/**
 * Render a unified diff for a file edit as RAW markdown (a ```diff fenced
 * block). Escaping/splitting is handled downstream by the markdown converter.
 */
import { structuredPatch } from "diff";

export interface DiffInput {
  path: string;
  oldText: string | null | undefined;
  newText: string | null | undefined;
  maxLines: number;
}

export interface DiffResult {
  block: string; // raw ```diff fenced markdown, or ""
  added: number;
  removed: number;
}

export function renderUnifiedDiff(input: DiffInput): DiffResult {
  const oldText = input.oldText ?? "";
  const newText = input.newText ?? "";
  if (oldText === newText) return { block: "", added: 0, removed: 0 };

  const patch = structuredPatch(input.path, input.path, oldText, newText, "", "", { context: 2 });
  const lines: string[] = [];
  let added = 0;
  let removed = 0;

  for (const hunk of patch.hunks) {
    lines.push(`@@ -${hunk.oldStart},${hunk.oldLines} +${hunk.newStart},${hunk.newLines} @@`);
    for (const l of hunk.lines) {
      if (l.startsWith("\\")) continue; // drop "\ No newline at end of file"
      if (l.startsWith("+")) added++;
      else if (l.startsWith("-")) removed++;
      lines.push(l);
    }
  }
  if (lines.length === 0) return { block: "", added, removed };

  // Display-only: keep head + tail so long edits still show both the start and
  // the end of the change. Full content remains in the agent session context.
  let shown = lines;
  let note = "";
  if (lines.length > input.maxLines) {
    const headN = Math.max(1, Math.floor(input.maxLines * 0.45));
    const tailN = Math.max(1, input.maxLines - headN);
    const omitted = lines.length - headN - tailN;
    shown = [
      ...lines.slice(0, headN),
      `… (${omitted} lines omitted) …`,
      ...lines.slice(lines.length - tailN),
    ];
    note = "";
  }
  return { block: "```diff\n" + shown.join("\n") + note + "\n```", added, removed };
}