#!/usr/bin/env bash
set -euo pipefail

# detect-target-branch
#
# Determines the target branch for a PR of the current branch.
#
# Uses git name-rev to find the nearest ref (branch/tag) that is an ancestor of
# HEAD, falling back to a graph walk if name-rev returns the current branch.
#
# Pure local — no remote, no SSH, no network. Works even when the current
# branch shares its tip commit with the target (not yet diverged).
#
# Usage:
#   tools/detect-target-branch             # print branch name
#   tools/detect-target-branch --verbose   # print "branch~N"
#
# Exit status:
#   0 — target found
#   1 — no target found (orphan commit, no other branches)

verbose=false

for arg do
  case "$arg" in
    --verbose|-v) verbose=true ;;
    --name-only)  ;;
    --help|-h)
      sed -n '/^#/,/^$/p' "$0" | sed 's/^# //; s/^#$//'
      exit 0
      ;;
    *)
      echo "Unknown option: $arg" >&2
      echo "Usage: $(basename "$0") [--verbose|-v] [--name-only] [--help|-h]" >&2
      exit 1
      ;;
  esac
done

current=$(git rev-parse --abbrev-ref HEAD)

# Use name-rev to find the nearest ref — it has smart tie-breaking
raw=$(git name-rev --name-only HEAD 2>/dev/null)
target=${raw%%~[0-9]*}  # strip ~N distance suffix

# If name-rev returned the current branch itself, walk backwards
if [ "$target" = "$current" ] || [ -z "$target" ]; then
  target=$(git rev-list HEAD | while read commit; do
    branch=$(git branch --points-at="$commit" --format='%(refname:short)' 2>/dev/null \
      | grep -v "^$current$" \
      | head -1 || true)
    if [ -n "$branch" ]; then
      echo "$branch"
      break
    fi
  done || true)

  if [ -z "$target" ]; then
    echo "error: unable to determine target branch" >&2
    exit 1
  fi
fi

if [ "$verbose" = true ]; then
  target_commit=$(git rev-parse "$target" 2>/dev/null || true)
  if [ -n "$target_commit" ]; then
    distance=$(git rev-list --count HEAD ^"$target_commit" 2>/dev/null || echo "0")
    echo "${target}~${distance}"
  else
    echo "${target}~?"
  fi
else
  echo "$target"
fi