Function computeFileModificationState

src/utils/commitAttribution.ts:326–381

* Compute the character contribution for a file modification. * Returns the FileAttributionState to store, or null if tracking failed.

(
  existingFileStates: Map<string, FileAttributionState>,
  filePath: string,
  oldContent: string,
  newContent: string,
  mtime: number,
)

 * Returns the FileAttributionState to store, or null if tracking failed.
 */
function computeFileModificationState(
  existingFileStates: Map<string, FileAttributionState>,
  filePath: string,
  oldContent: string,
  newContent: string,
  mtime: number,
): FileAttributionState | null {
  const normalizedPath = normalizeFilePath(filePath)

  try {
    // Calculate Claude's character contribution
    let claudeContribution: number

    if (oldContent === '' || newContent === '') {
      // New file or full deletion - contribution is the content length
      claudeContribution =
        oldContent === '' ? newContent.length : oldContent.length
    } else {
      // Find actual changed region via common prefix/suffix matching.
      // This correctly handles same-length replacements (e.g., "Esc" → "esc")
      // where Math.abs(newLen - oldLen) would be 0.
      const minLen = Math.min(oldContent.length, newContent.length)
      let prefixEnd = 0
      while (
        prefixEnd < minLen &&
        oldContent[prefixEnd] === newContent[prefixEnd]
      ) {
        prefixEnd++
      }
      let suffixLen = 0
      while (
        suffixLen < minLen - prefixEnd &&
        oldContent[oldContent.length - 1 - suffixLen] ===
          newContent[newContent.length - 1 - suffixLen]
      ) {
        suffixLen++
      }
      const oldChangedLen = oldContent.length - prefixEnd - suffixLen
      const newChangedLen = newContent.length - prefixEnd - suffixLen
      claudeContribution = Math.max(oldChangedLen, newChangedLen)
    }

    // Get current file state if it exists
    const existingState = existingFileStates.get(normalizedPath)
    const existingContribution = existingState?.claudeContribution ?? 0

    return {
      contentHash: computeContentHash(newContent),
      claudeContribution: existingContribution + claudeContribution,
      mtime,
    }
  } catch (error) {
    logError(error as Error)
    return null
  }
}

/**