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Generate SourceMap mappings string using array of character codes and fewer String.fromCharCode calls #44031

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Merged
merged 3 commits into from
May 18, 2021
Merged

Generate SourceMap mappings string using array of character codes and fewer String.fromCharCode calls #44031

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21e8ac0
Test normal char code array for source mappings
dmichon-msft May 10, 2021
228f3eb
Limit buffer size, minor performance tweaks
dmichon-msft May 11, 2021
0d5b285
Always commit at exactly chunk size
dmichon-msft May 11, 2021
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88 changes: 52 additions & 36 deletions src/compiler/sourcemap.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ namespace ts {

const names: string[] = [];
let nameToNameIndexMap: ESMap<string, number> | undefined;
const mappingCharCodes: number[] = [];
let mappings = "";

// Last recorded and encoded mappings
Expand Down Expand Up @@ -210,6 +211,15 @@ namespace ts {
|| lastNameIndex !== pendingNameIndex;
}

function appendMappingCharCode(charCode: number) {
mappingCharCodes.push(charCode);
// String.fromCharCode accepts its arguments on the stack, so we have to chunk the input,
// otherwise we can get stack overflows for large source maps
if (mappingCharCodes.length >= 1024) {
flushMappingBuffer();
}
}

function commitPendingMapping() {
if (!hasPending || !shouldCommitMapping()) {
return;
Expand All @@ -221,40 +231,41 @@ namespace ts {
if (lastGeneratedLine < pendingGeneratedLine) {
// Emit line delimiters
do {
mappings += ";";
appendMappingCharCode(CharacterCodes.semicolon);
lastGeneratedLine++;
lastGeneratedCharacter = 0;
}
while (lastGeneratedLine < pendingGeneratedLine);
// Only need to set this once
lastGeneratedCharacter = 0;
}
else {
Debug.assertEqual(lastGeneratedLine, pendingGeneratedLine, "generatedLine cannot backtrack");
// Emit comma to separate the entry
if (hasLast) {
mappings += ",";
appendMappingCharCode(CharacterCodes.comma);
}
}

// 1. Relative generated character
mappings += base64VLQFormatEncode(pendingGeneratedCharacter - lastGeneratedCharacter);
appendBase64VLQ(pendingGeneratedCharacter - lastGeneratedCharacter);
lastGeneratedCharacter = pendingGeneratedCharacter;

if (hasPendingSource) {
// 2. Relative sourceIndex
mappings += base64VLQFormatEncode(pendingSourceIndex - lastSourceIndex);
appendBase64VLQ(pendingSourceIndex - lastSourceIndex);
lastSourceIndex = pendingSourceIndex;

// 3. Relative source line
mappings += base64VLQFormatEncode(pendingSourceLine - lastSourceLine);
appendBase64VLQ(pendingSourceLine - lastSourceLine);
lastSourceLine = pendingSourceLine;

// 4. Relative source character
mappings += base64VLQFormatEncode(pendingSourceCharacter - lastSourceCharacter);
appendBase64VLQ(pendingSourceCharacter - lastSourceCharacter);
lastSourceCharacter = pendingSourceCharacter;

if (hasPendingName) {
// 5. Relative nameIndex
mappings += base64VLQFormatEncode(pendingNameIndex - lastNameIndex);
appendBase64VLQ(pendingNameIndex - lastNameIndex);
lastNameIndex = pendingNameIndex;
}
}
Expand All @@ -263,8 +274,16 @@ namespace ts {
exit();
}

function flushMappingBuffer(): void {
if (mappingCharCodes.length > 0) {
mappings += String.fromCharCode.apply(undefined, mappingCharCodes);
mappingCharCodes.length = 0;
}
}

function toJSON(): RawSourceMap {
commitPendingMapping();
flushMappingBuffer();
return {
version: 3,
file,
Expand All @@ -275,6 +294,31 @@ namespace ts {
sourcesContent,
};
}

function appendBase64VLQ(inValue: number): void {
// Add a new least significant bit that has the sign of the value.
// if negative number the least significant bit that gets added to the number has value 1
// else least significant bit value that gets added is 0
// eg. -1 changes to binary : 01 [1] => 3
// +1 changes to binary : 01 [0] => 2
if (inValue < 0) {
inValue = ((-inValue) << 1) + 1;
}
else {
inValue = inValue << 1;
}

// Encode 5 bits at a time starting from least significant bits
do {
let currentDigit = inValue & 31; // 11111
inValue = inValue >> 5;
if (inValue > 0) {
// There are still more digits to decode, set the msb (6th bit)
currentDigit = currentDigit | 32;
}
appendMappingCharCode(base64FormatEncode(currentDigit));
} while (inValue > 0);
}
}

// Sometimes tools can see the following line as a source mapping url comment, so we mangle it a bit (the [M])
Expand Down Expand Up @@ -544,34 +588,6 @@ namespace ts {
-1;
}

function base64VLQFormatEncode(inValue: number) {
// Add a new least significant bit that has the sign of the value.
// if negative number the least significant bit that gets added to the number has value 1
// else least significant bit value that gets added is 0
// eg. -1 changes to binary : 01 [1] => 3
// +1 changes to binary : 01 [0] => 2
if (inValue < 0) {
inValue = ((-inValue) << 1) + 1;
}
else {
inValue = inValue << 1;
}

// Encode 5 bits at a time starting from least significant bits
let encodedStr = "";
do {
let currentDigit = inValue & 31; // 11111
inValue = inValue >> 5;
if (inValue > 0) {
// There are still more digits to decode, set the msb (6th bit)
currentDigit = currentDigit | 32;
}
encodedStr = encodedStr + String.fromCharCode(base64FormatEncode(currentDigit));
} while (inValue > 0);

return encodedStr;
}

interface MappedPosition {
generatedPosition: number;
source: string | undefined;
Expand Down