Optimize source map character offset calculation with correct UTF-16 counting

[Copilot]

GetECMALineAndCharacterOfPosition called utf8.RuneCountInString from line start to position for every source map mapping — O(n) per call, O(n×m) per file. This is the root cause of the reported 325s compile times. Additionally, rune count is wrong per the source map v3 spec which requires UTF-16 code units.

Fix

• Fast path: For ASCII-only segments (vast majority of source code), character offset = byte offset difference. A tight byte scan checks for non-ASCII and returns len(s) directly.
• Slow path: For non-ASCII content, properly counts UTF-16 code units including surrogate pairs for supplementary characters (U+10000+), which need 2 code units rather than the 1 that rune counting produced.

// Before: O(n) rune decode per call, incorrect for supplementary chars
character = utf8.RuneCountInString(sourceFile.Text()[lineMap[line]:pos])

// After: O(1) for ASCII lines, correct UTF-16 counting for non-ASCII
character = utf16Length(sourceFile.Text()[lineMap[line]:pos])

Benchmark

Added BenchmarkEmit exercising the full emit path with source maps enabled against the TypeScript compiler source.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>tsgo takes forever to compile, 325seconds vs typescript 42seconds</issue_title>
<issue_description>output:

  > tsgo --watch
  
  build starting at 08:53:43 AM
  build finished in 325.396s

typescript:

  [09:01:43] Starting compilation in watch mode...

  [09:02:25] Found 0 errors. Watching for file changes.

How can I help to fix this? what do you need? (Our codebase is not public)
</issue_description>

<agent_instructions>Source map calculation is extremely slow due to needing to encode utf-16 offsets. Write an emit benchmark which shows this issue, commit that, then come up with something that makes this problem go away.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@jakebailey Try without watch mode; watch mode is a prototype.

Use --diagnostics to get timings.

Use --pprofDir . to get profiles and upload them here.

But the code would be the most useful.</comment_new>
<comment_new>@jakebailey
Ah, I bet this is a dupe of #2380; can you make sure you're on 7.0.0-dev.20251217.1?</comment_new>
<comment_new>@jakebailey
No, I'm wrong, it's actually source map slowness.

[Profile](https://pprof.me/7596184f9f5b02e7b09e207d9ae9f2fc/?profileType=profile%3Acpu%3Ananoseconds%3Acpu%3Ananoseconds%3Adelta&color_by=filename&cur_path=%5B%7B%22functionName%22%3A%22sync.%28*WaitGroup%29.Go.func1%22%2C%22systemName%22%3A%22sync.%28*WaitGroup%29.Go.func1%22%2C%22fileName%22%3A%22D%3A%5C%5Crepos%5C%5Cmlog%5C%5Cmcc%5C%5CHTML%5C%5Cnode_modules%5C%5C%40typescript%5C%5Cnative-preview-win32-x64%5C%5Clib%5C%5Ctsgo.exe%22%2C%22lineNumber%22%3A237%2C%22address%22%3A%220x7ff7168976e9%22%2C%22inlined%22%3Afalse%7D%2C%7B%22functionName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcore.%28*parallelWorkGroup%29.Queue.func1%22%2C%22systemName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcore.%28*parallelWorkGroup%29.Queue.func1%22%2C%22fileName%22%3A%22D%3A%5C%5Crepos%5C%5Cmlog%5C%5Cmcc%5C%5CHTML%5C%5Cnode_modules%5C%5C%40typescript%5C%5Cnative-preview-win32-x64%5C%5Clib%5C%5Ctsgo.exe%22%2C%22lineNumber%22%3A39%2C%22address%22%3A%220x7ff716993012%22%2C%22inlined%22%3Afalse%7D%2C%7B%22functionName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcompiler.%28*Program%29.Emit.func2%22%2C%22systemName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcompiler.%28*Program%29.Emit.func2%22%2C%22fileName%22%3A%22D%3A%5C%5Crepos%5C%5Cmlog%5C%5Cmcc%5C%5CHTML%5C%5Cnode_modules%5C%5C%40typescript%5C%5Cnative-preview-win32-x64%5C%5Clib%5C%5Ctsgo.exe%22%2C%22lineNumber%22%3A1396%2C%22address%22%3A%220x7ff716d5cb37%22%2C%22inlined%22%3Afalse%7D%2C%7B%22functionName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcompiler.%28*emitter%29.emit%22%2C%22systemName%22%3A%22github.com%2Fmicrosoft%2Ftypescript-go%2Finternal%2Fcompiler.%28*emitter%29.emit%22%2C%22fileName%22%3A%22D%3A%5C%5Crepos%5C%5Cmlog%5C%5Cmcc%5C%5CHTML%5C%5Cnode_modules%5C%5C%40typescript%5C%5Cnative-preview-win32-x64%5C%5Clib%5C%5Ctsgo.exe%22%2C%22lineNumber%22%3A45%2C%22address%22%3A%22%22%2C%22...</comment_new>
<comment_new>@jakebailey
All in all, we need a repro of this issue that isn't a private codebase</comment_new>
<comment_new>@jakebailey
I'm sure we can come up with a better way to do this, this code hasn't been optimized. It was never an issue in the old compiler since strings were already UTF-16.</comment_new>

• Fixes tsgo takes forever to compile, 325seconds vs typescript 42seconds #2402

---

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[DanielRosenwasser]

Don't we have code like this somewhere?

[jakebailey]

It was in my weird regex parsing PR; I don't think this PR is even correct, though, because it's still O(n); I was hoping it'd generate a nice data structure.

[DanielRosenwasser]

That's what I figured as well, since decoding probably has a fast path anyway?

[DanielRosenwasser]

I think a cache for the previously-requested file and line of interest would be fine.

[jakebailey]

The gotcha is that each time the emitter does something, it updates the line map, which then goes back through this code (more or less)

[DanielRosenwasser]

The problem with this is you're already part of the way through the string, so if you have any non-ASCII on a long line, you have to go back and reinterpret all the previous contents.

Least invasive way to partially fix this is to keep things mostly as-is, but instead return len(s) + utf16LengthSlow([i:]) so we don't reiterate over the beginning of the string.

I think a better way to do this would be either

1. Keep 2 separate counters: one for the number of non-ASCII units, one for the number of bytes skipped to decode the current rune. At the end, you return len(s) + numNonAscii - numBytesSkipped.
2. You keep 1 counter nonAsciiAdjustments that just combines the two concepts above.

[jakebailey]

I'm going to close this, I think it's just not helpful in isolation