src/server/scriptVersionCache.ts

/*@internal*/
namespace ts.server {
    const lineCollectionCapacity = 4;

    interface LineCollection {
        charCount(): number;
        lineCount(): number;
        isLeaf(): this is LineLeaf;
        walk(rangeStart: number, rangeLength: number, walkFns: LineIndexWalker): void;
    }

    export interface AbsolutePositionAndLineText {
        absolutePosition: number;
        lineText: string | undefined;
    }

    const enum CharRangeSection {
        PreStart,
        Start,
        Entire,
        Mid,
        End,
        PostEnd
    }

    interface LineIndexWalker {
        goSubtree: boolean;
        done: boolean;
        leaf(relativeStart: number, relativeLength: number, lineCollection: LineLeaf): void;
        pre?(relativeStart: number, relativeLength: number, lineCollection: LineCollection,
            parent: LineNode, nodeType: CharRangeSection): void;
        post?(relativeStart: number, relativeLength: number, lineCollection: LineCollection,
            parent: LineNode, nodeType: CharRangeSection): void;
    }

    class EditWalker implements LineIndexWalker {
        goSubtree = true;
        get done() { return false; }

        readonly lineIndex = new LineIndex();
        // path to start of range
        private readonly startPath: LineCollection[];
        private readonly endBranch: LineCollection[] = [];
        private branchNode: LineNode | undefined;
        // path to current node
        private readonly stack: LineNode[];
        private state = CharRangeSection.Entire;
        private lineCollectionAtBranch: LineCollection | undefined;
        private initialText = "";
        private trailingText = "";

        constructor() {
            this.lineIndex.root = new LineNode();
            this.startPath = [this.lineIndex.root];
            this.stack = [this.lineIndex.root];
        }

        insertLines(insertedText: string | undefined, suppressTrailingText: boolean) {
            if (suppressTrailingText) {
                this.trailingText = "";
            }
            if (insertedText) {
                insertedText = this.initialText + insertedText + this.trailingText;
            }
            else {
                insertedText = this.initialText + this.trailingText;
            }
            const lm = LineIndex.linesFromText(insertedText);
            const lines = lm.lines;
            if (lines.length > 1 && lines[lines.length - 1] === "") {
                lines.pop();
            }
            let branchParent: LineNode | undefined;
            let lastZeroCount: LineCollection | undefined;

            for (let k = this.endBranch.length - 1; k >= 0; k--) {
                (this.endBranch[k] as LineNode).updateCounts();
                if (this.endBranch[k].charCount() === 0) {
                    lastZeroCount = this.endBranch[k];
                    if (k > 0) {
                        branchParent = this.endBranch[k - 1] as LineNode;
                    }
                    else {
                        branchParent = this.branchNode;
                    }
                }
            }
            if (lastZeroCount) {
                branchParent!.remove(lastZeroCount);
            }

            // path at least length two (root and leaf)
            const leafNode = this.startPath[this.startPath.length - 1] as LineLeaf;

            if (lines.length > 0) {
                leafNode.text = lines[0];

                if (lines.length > 1) {
                    let insertedNodes = new Array(lines.length - 1) as LineCollection[];
                    let startNode = leafNode as LineCollection;
                    for (let i = 1; i < lines.length; i++) {
                        insertedNodes[i - 1] = new LineLeaf(lines[i]);
                    }
                    let pathIndex = this.startPath.length - 2;
                    while (pathIndex >= 0) {
                        const insertionNode = this.startPath[pathIndex] as LineNode;
                        insertedNodes = insertionNode.insertAt(startNode, insertedNodes);
                        pathIndex--;
                        startNode = insertionNode;
                    }
                    let insertedNodesLen = insertedNodes.length;
                    while (insertedNodesLen > 0) {
                        const newRoot = new LineNode();
                        newRoot.add(this.lineIndex.root);
                        insertedNodes = newRoot.insertAt(this.lineIndex.root, insertedNodes);
                        insertedNodesLen = insertedNodes.length;
                        this.lineIndex.root = newRoot;
                    }
                    this.lineIndex.root.updateCounts();
                }
                else {
                    for (let j = this.startPath.length - 2; j >= 0; j--) {
                        (this.startPath[j] as LineNode).updateCounts();
                    }
                }
            }
            else {
                const insertionNode = this.startPath[this.startPath.length - 2] as LineNode;
                // no content for leaf node, so delete it
                insertionNode.remove(leafNode);
                for (let j = this.startPath.length - 2; j >= 0; j--) {
                    (this.startPath[j] as LineNode).updateCounts();
                }
            }

            return this.lineIndex;
        }

        post(_relativeStart: number, _relativeLength: number, lineCollection: LineCollection): void {
            // have visited the path for start of range, now looking for end
            // if range is on single line, we will never make this state transition
            if (lineCollection === this.lineCollectionAtBranch) {
                this.state = CharRangeSection.End;
            }
            // always pop stack because post only called when child has been visited
            this.stack.pop();
        }

        pre(_relativeStart: number, _relativeLength: number, lineCollection: LineCollection, _parent: LineCollection, nodeType: CharRangeSection): void {
            // currentNode corresponds to parent, but in the new tree
            const currentNode = this.stack[this.stack.length - 1];

            if ((this.state === CharRangeSection.Entire) && (nodeType === CharRangeSection.Start)) {
                // if range is on single line, we will never make this state transition
                this.state = CharRangeSection.Start;
                this.branchNode = currentNode;
                this.lineCollectionAtBranch = lineCollection;
            }

            let child: LineCollection | undefined;
            function fresh(node: LineCollection): LineCollection {
                if (node.isLeaf()) {
                    return new LineLeaf("");
                }
                else return new LineNode();
            }
            switch (nodeType) {
                case CharRangeSection.PreStart:
                    this.goSubtree = false;
                    if (this.state !== CharRangeSection.End) {
                        currentNode.add(lineCollection);
                    }
                    break;
                case CharRangeSection.Start:
                    if (this.state === CharRangeSection.End) {
                        this.goSubtree = false;
                    }
                    else {
                        child = fresh(lineCollection);
                        currentNode.add(child);
                        this.startPath.push(child);
                    }
                    break;
                case CharRangeSection.Entire:
                    if (this.state !== CharRangeSection.End) {
                        child = fresh(lineCollection);
                        currentNode.add(child);
                        this.startPath.push(child);
                    }
                    else {
                        if (!lineCollection.isLeaf()) {
                            child = fresh(lineCollection);
                            currentNode.add(child);
                            this.endBranch.push(child);
                        }
                    }
                    break;
                case CharRangeSection.Mid:
                    this.goSubtree = false;
                    break;
                case CharRangeSection.End:
                    if (this.state !== CharRangeSection.End) {
                        this.goSubtree = false;
                    }
                    else {
                        if (!lineCollection.isLeaf()) {
                            child = fresh(lineCollection);
                            currentNode.add(child);
                            this.endBranch.push(child);
                        }
                    }
                    break;
                case CharRangeSection.PostEnd:
                    this.goSubtree = false;
                    if (this.state !== CharRangeSection.Start) {
                        currentNode.add(lineCollection);
                    }
                    break;
            }
            if (this.goSubtree) {
                this.stack.push(child as LineNode);
            }
        }
        // just gather text from the leaves
        leaf(relativeStart: number, relativeLength: number, ll: LineLeaf) {
            if (this.state === CharRangeSection.Start) {
                this.initialText = ll.text.substring(0, relativeStart);
            }
            else if (this.state === CharRangeSection.Entire) {
                this.initialText = ll.text.substring(0, relativeStart);
                this.trailingText = ll.text.substring(relativeStart + relativeLength);
            }
            else {
                // state is CharRangeSection.End
                this.trailingText = ll.text.substring(relativeStart + relativeLength);
            }
        }
    }

    // text change information
    class TextChange {
        constructor(public pos: number, public deleteLen: number, public insertedText?: string) {
        }

        getTextChangeRange() {
            return createTextChangeRange(createTextSpan(this.pos, this.deleteLen),
                this.insertedText ? this.insertedText.length : 0);
        }
    }

    export class ScriptVersionCache {
        private changes: TextChange[] = [];
        private readonly versions: LineIndexSnapshot[] = new Array<LineIndexSnapshot>(ScriptVersionCache.maxVersions);
        private minVersion = 0; // no versions earlier than min version will maintain change history

        private currentVersion = 0;

        private static readonly changeNumberThreshold = 8;
        private static readonly changeLengthThreshold = 256;
        private static readonly maxVersions = 8;

        private versionToIndex(version: number) {
            if (version < this.minVersion || version > this.currentVersion) {
                return undefined;
            }
            return version % ScriptVersionCache.maxVersions;
        }

        private currentVersionToIndex() {
            return this.currentVersion % ScriptVersionCache.maxVersions;
        }

        // REVIEW: can optimize by coalescing simple edits
        edit(pos: number, deleteLen: number, insertedText?: string) {
            this.changes.push(new TextChange(pos, deleteLen, insertedText));
            if (this.changes.length > ScriptVersionCache.changeNumberThreshold ||
                deleteLen > ScriptVersionCache.changeLengthThreshold ||
                insertedText && insertedText.length > ScriptVersionCache.changeLengthThreshold) {
                this.getSnapshot();
            }
        }

        getSnapshot(): IScriptSnapshot { return this._getSnapshot(); }

        private _getSnapshot(): LineIndexSnapshot {
            let snap = this.versions[this.currentVersionToIndex()];
            if (this.changes.length > 0) {
                let snapIndex = snap.index;
                for (const change of this.changes) {
                    snapIndex = snapIndex.edit(change.pos, change.deleteLen, change.insertedText);
                }
                snap = new LineIndexSnapshot(this.currentVersion + 1, this, snapIndex, this.changes);

                this.currentVersion = snap.version;
                this.versions[this.currentVersionToIndex()] = snap;
                this.changes = [];

                if ((this.currentVersion - this.minVersion) >= ScriptVersionCache.maxVersions) {
                    this.minVersion = (this.currentVersion - ScriptVersionCache.maxVersions) + 1;
                }
            }
            return snap;
        }

        getSnapshotVersion(): number {
            return this._getSnapshot().version;
        }

        getAbsolutePositionAndLineText(oneBasedLine: number): AbsolutePositionAndLineText {
            return this._getSnapshot().index.lineNumberToInfo(oneBasedLine);
        }

        lineOffsetToPosition(line: number, column: number): number {
            return this._getSnapshot().index.absolutePositionOfStartOfLine(line) + (column - 1);
        }

        positionToLineOffset(position: number): protocol.Location {
            return this._getSnapshot().index.positionToLineOffset(position);
        }

        lineToTextSpan(line: number): TextSpan {
            const index = this._getSnapshot().index;
            const { lineText, absolutePosition } = index.lineNumberToInfo(line + 1);
            const len = lineText !== undefined ? lineText.length : index.absolutePositionOfStartOfLine(line + 2) - absolutePosition;
            return createTextSpan(absolutePosition, len);
        }

        getTextChangesBetweenVersions(oldVersion: number, newVersion: number) {
            if (oldVersion < newVersion) {
                if (oldVersion >= this.minVersion) {
                    const textChangeRanges: TextChangeRange[] = [];
                    for (let i = oldVersion + 1; i <= newVersion; i++) {
                        const snap = this.versions[this.versionToIndex(i)!]; // TODO: GH#18217
                        for (const textChange of snap.changesSincePreviousVersion) {
                            textChangeRanges.push(textChange.getTextChangeRange());
                        }
                    }
                    return collapseTextChangeRangesAcrossMultipleVersions(textChangeRanges);
                }
                else {
                    return undefined;
                }
            }
            else {
                return unchangedTextChangeRange;
            }
        }

        getLineCount() {
            return this._getSnapshot().index.getLineCount();
        }

        static fromString(script: string) {
            const svc = new ScriptVersionCache();
            const snap = new LineIndexSnapshot(0, svc, new LineIndex());
            svc.versions[svc.currentVersion] = snap;
            const lm = LineIndex.linesFromText(script);
            snap.index.load(lm.lines);
            return svc;
        }
    }

    class LineIndexSnapshot implements IScriptSnapshot {
        constructor(readonly version: number, readonly cache: ScriptVersionCache, readonly index: LineIndex, readonly changesSincePreviousVersion: readonly TextChange[] = emptyArray) {
        }

        getText(rangeStart: number, rangeEnd: number) {
            return this.index.getText(rangeStart, rangeEnd - rangeStart);
        }

        getLength() {
            return this.index.getLength();
        }

        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange | undefined {
            if (oldSnapshot instanceof LineIndexSnapshot && this.cache === oldSnapshot.cache) {
                if (this.version <= oldSnapshot.version) {
                    return unchangedTextChangeRange;
                }
                else {
                    return this.cache.getTextChangesBetweenVersions(oldSnapshot.version, this.version);
                }
            }
        }
    }

    export class LineIndex {
        root!: LineNode;
        // set this to true to check each edit for accuracy
        checkEdits = false;

        absolutePositionOfStartOfLine(oneBasedLine: number): number {
            return this.lineNumberToInfo(oneBasedLine).absolutePosition;
        }

        positionToLineOffset(position: number): protocol.Location {
            const { oneBasedLine, zeroBasedColumn } = this.root.charOffsetToLineInfo(1, position);
            return { line: oneBasedLine, offset: zeroBasedColumn + 1 };
        }

        private positionToColumnAndLineText(position: number): { zeroBasedColumn: number, lineText: string | undefined } {
            return this.root.charOffsetToLineInfo(1, position);
        }

        getLineCount() {
            return this.root.lineCount();
        }

        lineNumberToInfo(oneBasedLine: number): AbsolutePositionAndLineText {
            const lineCount = this.getLineCount();
            if (oneBasedLine <= lineCount) {
                const { position, leaf } = this.root.lineNumberToInfo(oneBasedLine, 0);
                return { absolutePosition: position, lineText: leaf && leaf.text };
            }
            else {
                return { absolutePosition: this.root.charCount(), lineText: undefined };
            }
        }

        load(lines: string[]) {
            if (lines.length > 0) {
                const leaves: LineLeaf[] = [];
                for (let i = 0; i < lines.length; i++) {
                    leaves[i] = new LineLeaf(lines[i]);
                }
                this.root = LineIndex.buildTreeFromBottom(leaves);
            }
            else {
                this.root = new LineNode();
            }
        }

        walk(rangeStart: number, rangeLength: number, walkFns: LineIndexWalker) {
            this.root.walk(rangeStart, rangeLength, walkFns);
        }

        getText(rangeStart: number, rangeLength: number) {
            let accum = "";
            if ((rangeLength > 0) && (rangeStart < this.root.charCount())) {
                this.walk(rangeStart, rangeLength, {
                    goSubtree: true,
                    done: false,
                    leaf: (relativeStart: number, relativeLength: number, ll: LineLeaf) => {
                        accum = accum.concat(ll.text.substring(relativeStart, relativeStart + relativeLength));
                    }
                });
            }
            return accum;
        }

        getLength(): number {
            return this.root.charCount();
        }

        every(f: (ll: LineLeaf, s: number, len: number) => boolean, rangeStart: number, rangeEnd?: number) {
            if (!rangeEnd) {
                rangeEnd = this.root.charCount();
            }
            const walkFns = {
                goSubtree: true,
                done: false,
                leaf(this: LineIndexWalker, relativeStart: number, relativeLength: number, ll: LineLeaf) {
                    if (!f(ll, relativeStart, relativeLength)) {
                        this.done = true;
                    }
                }
            };
            this.walk(rangeStart, rangeEnd - rangeStart, walkFns);
            return !walkFns.done;
        }

        edit(pos: number, deleteLength: number, newText?: string): LineIndex {
            if (this.root.charCount() === 0) {
                Debug.assert(deleteLength === 0); // Can't delete from empty document
                if (newText !== undefined) {
                    this.load(LineIndex.linesFromText(newText).lines);
                    return this;
                }
                return undefined!; // TODO: GH#18217
            }
            else {
                let checkText: string | undefined;
                if (this.checkEdits) {
                    const source = this.getText(0, this.root.charCount());
                    checkText = source.slice(0, pos) + newText + source.slice(pos + deleteLength);
                }
                const walker = new EditWalker();
                let suppressTrailingText = false;
                if (pos >= this.root.charCount()) {
                    // insert at end
                    pos = this.root.charCount() - 1;
                    const endString = this.getText(pos, 1);
                    if (newText) {
                        newText = endString + newText;
                    }
                    else {
                        newText = endString;
                    }
                    deleteLength = 0;
                    suppressTrailingText = true;
                }
                else if (deleteLength > 0) {
                    // check whether last characters deleted are line break
                    const e = pos + deleteLength;
                    const { zeroBasedColumn, lineText } = this.positionToColumnAndLineText(e);
                    if (zeroBasedColumn === 0) {
                        // move range end just past line that will merge with previous line
                        deleteLength += lineText!.length; // TODO: GH#18217
                        // store text by appending to end of insertedText
                        newText = newText ? newText + lineText : lineText;
                    }
                }

                this.root.walk(pos, deleteLength, walker);
                walker.insertLines(newText, suppressTrailingText);

                if (this.checkEdits) {
                    const updatedText = walker.lineIndex.getText(0, walker.lineIndex.getLength());
                    Debug.assert(checkText === updatedText, "buffer edit mismatch");
                }

                return walker.lineIndex;
            }
        }

        private static buildTreeFromBottom(nodes: LineCollection[]): LineNode {
            if (nodes.length < lineCollectionCapacity) {
                return new LineNode(nodes);
            }

            const interiorNodes = new Array<LineNode>(Math.ceil(nodes.length / lineCollectionCapacity));
            let nodeIndex = 0;
            for (let i = 0; i < interiorNodes.length; i++) {
                const end = Math.min(nodeIndex + lineCollectionCapacity, nodes.length);
                interiorNodes[i] = new LineNode(nodes.slice(nodeIndex, end));
                nodeIndex = end;
            }
            return this.buildTreeFromBottom(interiorNodes);
        }

        static linesFromText(text: string) {
            const lineMap = computeLineStarts(text);

            if (lineMap.length === 0) {
                return { lines: [] as string[], lineMap };
            }
            const lines = new Array(lineMap.length) as string[];
            const lc = lineMap.length - 1;
            for (let lmi = 0; lmi < lc; lmi++) {
                lines[lmi] = text.substring(lineMap[lmi], lineMap[lmi + 1]);
            }

            const endText = text.substring(lineMap[lc]);
            if (endText.length > 0) {
                lines[lc] = endText;
            }
            else {
                lines.pop();
            }
            return { lines, lineMap };
        }
    }

    class LineNode implements LineCollection {
        totalChars = 0;
        totalLines = 0;

        constructor(private readonly children: LineCollection[] = []) {
            if (children.length) this.updateCounts();
        }

        isLeaf() {
            return false;
        }

        updateCounts() {
            this.totalChars = 0;
            this.totalLines = 0;
            for (const child of this.children) {
                this.totalChars += child.charCount();
                this.totalLines += child.lineCount();
            }
        }

        private execWalk(rangeStart: number, rangeLength: number, walkFns: LineIndexWalker, childIndex: number, nodeType: CharRangeSection) {
            if (walkFns.pre) {
                walkFns.pre(rangeStart, rangeLength, this.children[childIndex], this, nodeType);
            }
            if (walkFns.goSubtree) {
                this.children[childIndex].walk(rangeStart, rangeLength, walkFns);
                if (walkFns.post) {
                    walkFns.post(rangeStart, rangeLength, this.children[childIndex], this, nodeType);
                }
            }
            else {
                walkFns.goSubtree = true;
            }
            return walkFns.done;
        }

        private skipChild(relativeStart: number, relativeLength: number, childIndex: number, walkFns: LineIndexWalker, nodeType: CharRangeSection) {
            if (walkFns.pre && (!walkFns.done)) {
                walkFns.pre(relativeStart, relativeLength, this.children[childIndex], this, nodeType);
                walkFns.goSubtree = true;
            }
        }

        walk(rangeStart: number, rangeLength: number, walkFns: LineIndexWalker) {
            // assume (rangeStart < this.totalChars) && (rangeLength <= this.totalChars)
            let childIndex = 0;
            let childCharCount = this.children[childIndex].charCount();
            // find sub-tree containing start
            let adjustedStart = rangeStart;
            while (adjustedStart >= childCharCount) {
                this.skipChild(adjustedStart, rangeLength, childIndex, walkFns, CharRangeSection.PreStart);
                adjustedStart -= childCharCount;
                childIndex++;
                childCharCount = this.children[childIndex].charCount();
            }
            // Case I: both start and end of range in same subtree
            if ((adjustedStart + rangeLength) <= childCharCount) {
                if (this.execWalk(adjustedStart, rangeLength, walkFns, childIndex, CharRangeSection.Entire)) {
                    return;
                }
            }
            else {
                // Case II: start and end of range in different subtrees (possibly with subtrees in the middle)
                if (this.execWalk(adjustedStart, childCharCount - adjustedStart, walkFns, childIndex, CharRangeSection.Start)) {
                    return;
                }
                let adjustedLength = rangeLength - (childCharCount - adjustedStart);
                childIndex++;
                const child = this.children[childIndex];
                childCharCount = child.charCount();
                while (adjustedLength > childCharCount) {
                    if (this.execWalk(0, childCharCount, walkFns, childIndex, CharRangeSection.Mid)) {
                        return;
                    }
                    adjustedLength -= childCharCount;
                    childIndex++;
                    childCharCount = this.children[childIndex].charCount();
                }
                if (adjustedLength > 0) {
                    if (this.execWalk(0, adjustedLength, walkFns, childIndex, CharRangeSection.End)) {
                        return;
                    }
                }
            }
            // Process any subtrees after the one containing range end
            if (walkFns.pre) {
                const clen = this.children.length;
                if (childIndex < (clen - 1)) {
                    for (let ej = childIndex + 1; ej < clen; ej++) {
                        this.skipChild(0, 0, ej, walkFns, CharRangeSection.PostEnd);
                    }
                }
            }
        }

        // Input position is relative to the start of this node.
        // Output line number is absolute.
        charOffsetToLineInfo(lineNumberAccumulator: number, relativePosition: number): { oneBasedLine: number, zeroBasedColumn: number, lineText: string | undefined } {
            if (this.children.length === 0) {
                // Root node might have no children if this is an empty document.
                return { oneBasedLine: lineNumberAccumulator, zeroBasedColumn: relativePosition, lineText: undefined };
            }

            for (const child of this.children) {
                if (child.charCount() > relativePosition) {
                    if (child.isLeaf()) {
                        return { oneBasedLine: lineNumberAccumulator, zeroBasedColumn: relativePosition, lineText: child.text };
                    }
                    else {
                        return (child as LineNode).charOffsetToLineInfo(lineNumberAccumulator, relativePosition);
                    }
                }
                else {
                    relativePosition -= child.charCount();
                    lineNumberAccumulator += child.lineCount();
                }
            }

            // Skipped all children
            const lineCount = this.lineCount();
            if (lineCount === 0) { // it's empty! (and lineNumberToInfo expects a one-based line)
                return { oneBasedLine: 1, zeroBasedColumn: 0, lineText: undefined };
            }
            const leaf = Debug.checkDefined(this.lineNumberToInfo(lineCount, 0).leaf);
            return { oneBasedLine: lineCount, zeroBasedColumn: leaf.charCount(), lineText: undefined };
        }

        /**
         * Input line number is relative to the start of this node.
         * Output line number is relative to the child.
         * positionAccumulator will be an absolute position once relativeLineNumber reaches 0.
         */
        lineNumberToInfo(relativeOneBasedLine: number, positionAccumulator: number): { position: number, leaf: LineLeaf | undefined } {
            for (const child of this.children) {
                const childLineCount = child.lineCount();
                if (childLineCount >= relativeOneBasedLine) {
                    return child.isLeaf() ? { position: positionAccumulator, leaf: child } : (child as LineNode).lineNumberToInfo(relativeOneBasedLine, positionAccumulator);
                }
                else {
                    relativeOneBasedLine -= childLineCount;
                    positionAccumulator += child.charCount();
                }
            }

            return { position: positionAccumulator, leaf: undefined };
        }

        private splitAfter(childIndex: number) {
            let splitNode: LineNode | undefined;
            const clen = this.children.length;
            childIndex++;
            const endLength = childIndex;
            if (childIndex < clen) {
                splitNode = new LineNode();
                while (childIndex < clen) {
                    splitNode.add(this.children[childIndex]);
                    childIndex++;
                }
                splitNode.updateCounts();
            }
            this.children.length = endLength;
            return splitNode;
        }

        remove(child: LineCollection) {
            const childIndex = this.findChildIndex(child);
            const clen = this.children.length;
            if (childIndex < (clen - 1)) {
                for (let i = childIndex; i < (clen - 1); i++) {
                    this.children[i] = this.children[i + 1];
                }
            }
            this.children.pop();
        }

        private findChildIndex(child: LineCollection) {
            const childIndex = this.children.indexOf(child);
            Debug.assert(childIndex !== -1);
            return childIndex;
        }

        insertAt(child: LineCollection, nodes: LineCollection[]) {
            let childIndex = this.findChildIndex(child);
            const clen = this.children.length;
            const nodeCount = nodes.length;
            // if child is last and there is more room and only one node to place, place it
            if ((clen < lineCollectionCapacity) && (childIndex === (clen - 1)) && (nodeCount === 1)) {
                this.add(nodes[0]);
                this.updateCounts();
                return [];
            }
            else {
                const shiftNode = this.splitAfter(childIndex);
                let nodeIndex = 0;
                childIndex++;
                while ((childIndex < lineCollectionCapacity) && (nodeIndex < nodeCount)) {
                    this.children[childIndex] = nodes[nodeIndex];
                    childIndex++;
                    nodeIndex++;
                }
                let splitNodes: LineNode[] = [];
                let splitNodeCount = 0;
                if (nodeIndex < nodeCount) {
                    splitNodeCount = Math.ceil((nodeCount - nodeIndex) / lineCollectionCapacity);
                    splitNodes = new Array(splitNodeCount) as LineNode[];
                    let splitNodeIndex = 0;
                    for (let i = 0; i < splitNodeCount; i++) {
                        splitNodes[i] = new LineNode();
                    }
                    let splitNode = splitNodes[0];
                    while (nodeIndex < nodeCount) {
                        splitNode.add(nodes[nodeIndex]);
                        nodeIndex++;
                        if (splitNode.children.length === lineCollectionCapacity) {
                            splitNodeIndex++;
                            splitNode = splitNodes[splitNodeIndex];
                        }
                    }
                    for (let i = splitNodes.length - 1; i >= 0; i--) {
                        if (splitNodes[i].children.length === 0) {
                            splitNodes.pop();
                        }
                    }
                }
                if (shiftNode) {
                    splitNodes.push(shiftNode);
                }
                this.updateCounts();
                for (let i = 0; i < splitNodeCount; i++) {
                    splitNodes[i].updateCounts();
                }
                return splitNodes;
            }
        }

        // assume there is room for the item; return true if more room
        add(collection: LineCollection): void {
            this.children.push(collection);
            Debug.assert(this.children.length <= lineCollectionCapacity);
        }

        charCount() {
            return this.totalChars;
        }

        lineCount() {
            return this.totalLines;
        }
    }

    class LineLeaf implements LineCollection {
        constructor(public text: string) {
        }

        isLeaf() {
            return true;
        }

        walk(rangeStart: number, rangeLength: number, walkFns: LineIndexWalker) {
            walkFns.leaf(rangeStart, rangeLength, this);
        }

        charCount() {
            return this.text.length;
        }

        lineCount() {
            return 1;
        }
    }
}