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

# changelog-monotonic.sh [<base-ref>] [<changelog>] — assert that no SHIPPED
# release heading was DELETED by this branch: the set of '^## X.Y.Z' headings
# on HEAD must be a SUPERSET of the set at the merge base.
#
# The failure it exists to catch (#122, caught in review of #118) leaves no
# trace either. An author adding an entry under '## Unreleased' REPLACES the
# line below it instead of inserting above it:
#
#     -## 0.8.0 — 2026-07-19
#     +## Unreleased
#     +
#     +### Fixed
#     +
#     +- **An entry**
#
# git merges that cleanly — it is a one-line edit inside a file nobody has
# touched concurrently — and the shipped section's whole body is silently
# absorbed into '## Unreleased'. 0.8.0 no longer HAS a section; the notes
# anchor release-notes.sh extracts by is gone, and the next release cut from
# that state republishes 0.8.0's prose as if it were new.
#
# changelog-armed.sh is green on exactly that tree, correctly: it asks only
# whether the TOP section agrees with VERSION, and deleting '## 0.8.0' leaves
# '## Unreleased' on top. It is not wrong, it is narrow — it guards ONE
# heading, the one a PR is about to write under. This guards the REST of the
# file, the part no single tree can be asked about at all, because "a heading
# disappeared" is not a property of a tree — it is a property of a DIFF.
#
# The rule, and why it needs no tuning: release headings are APPEND-ONLY. The
# ceremony (#96) adds one and never removes one; nothing else in the documented
# flow (CONTRIBUTING.md, "Releases") touches them. So SUPERSET is exact — it
# has no legitimate violation to carve an exception for. The stamp is covered
# for free: rewriting '## Unreleased' -> '## X.Y.Z — DATE' ADDS X.Y.Z and
# removes no X.Y.Z heading, because 'Unreleased' is not one. '## Unreleased'
# is deliberately NOT in the set this guards — changelog-armed.sh owns that
# heading, keyed on VERSION, and the ceremony legitimately consumes it.
#
# A file of its own, NOT a clause inside changelog-armed.sh, for three
# reasons. Its input is different (a git history, not two files). Its
# degradation is different (no base ref is a SKIP, not a failure). And
# changelog-armed.sh is driven by test/release.sh against constructed
# two-file trees that are not git repos at all — folding a git-dependent
# assert into it would make every one of those cases either skip or lie.
# Same discipline as release-notes.sh: its own file so a test can drive it.

base_ref="${1:-${CHANGELOG_MONOTONIC_BASE:-origin/main}}"
changelog="${2:-CHANGELOG.md}"

# Fail-closed switch: CI sets it, so a SKIP that would be a sensible local
# degradation becomes a red run there instead. A guard that can silently
# stop guarding is the failure shape this whole family of checks exists to
# refuse, so the skip path is loud and CI refuses to take it at all.
strict="${CHANGELOG_MONOTONIC_STRICT:-0}"

skip() {
  if [ "$strict" = "1" ]; then
    echo "changelog-monotonic: $* — and CHANGELOG_MONOTONIC_STRICT=1, so this is a FAILURE, not a skip." >&2
    echo "  CI sets STRICT because a guard that quietly stops guarding is worse than no guard." >&2
    echo "  (Uniqueness on HEAD already passed; it is containment that cannot run.)" >&2
    echo "  Fix the checkout, not this script: the base ref must be fetched (fetch-depth: 0)." >&2
    exit 1
  fi
  echo "changelog-monotonic: containment SKIPPED — $*"
  echo "  (Uniqueness on HEAD already ran and passed — only the deleted-heading"
  echo "   half needs the history. In CI this same condition is a hard failure.)"
  exit 0
}

[ -f "$changelog" ] || { echo "changelog-monotonic: no such file: $changelog" >&2; exit 1; }

# The set of RELEASE headings: '## <token> ...' where <token> looks like a
# version. Field $2, the same split changelog-armed.sh and release-notes.sh
# use, so the three cannot disagree about what a section header is.
# 'Unreleased' fails the shape and is excluded by construction.
headings_raw() {
  awk '
    /^## / && $2 ~ /^[0-9]+\.[0-9]+\.[0-9]+/ { print $2 }
  '
}
headings() { headings_raw | sort -u; }

# --- uniqueness on HEAD (the #118 class) -------------------------------------
# Containment catches a DELETED heading. It cannot catch a DUPLICATED one: the
# duplicate is head-side SURPLUS, and `comm -23` (base minus head) is blind to
# extras on the head side — with or without `sort -u`, base {0.8.0} minus head
# {0.8.0, 0.8.0} is empty. Multiset comparison does not close it either, for
# the same reason. The assert that does is uniqueness of version headings ON
# HEAD, kept alongside containment rather than replacing it.
#
# This is the shape #118's bad rebase actually produced: two
# `## 0.8.0 — 2026-07-19` headings with the incoming entry between them. Every
# other guard stayed green — markers absent, changelog-armed.sh happy (the top
# section was still right), tests and shellcheck clean — while
# release-notes.sh re-armed its grab on the second heading and folded post-cut
# prose into the shipped release body.
#
# Nothing legitimate repeats a version heading: the ceremony stamps a NEW
# version, and 'Unreleased' fails the version shape and never reaches here.
dupes="$(headings_raw < "$changelog" | sort | uniq -d)"
if [ -n "$dupes" ]; then
  {
    echo "changelog-monotonic: $changelog has DUPLICATE release heading(s):"
    echo
    printf '%s\n' "$dupes" | sed 's/^/    ## /'
    echo
    cat <<EOF
  Each version heading must appear exactly once. A repeat splits one release
  into two same-named sections, and release-notes.sh re-arms its extraction on
  every matching '## ' line — so the published body for that version absorbs
  whatever sits between the copies, and an entry stranded there is dropped from
  the NEXT release's notes as well.

  This is the #118 shape: an entry meant for '## Unreleased' was inserted after
  a shipped heading, and the heading re-added below it. The fix is one heading,
  with the entry above it under '## Unreleased':

      ## Unreleased

      ### Fixed

      - **Your entry**

      ## $(printf '%s\n' "$dupes" | head -1) — DATE     <- exactly once

  Quick check on any changelog-touching rebase:

      diff <(git show origin/main:$changelog | grep '^## ') <(grep '^## ' $changelog)
EOF
  } >&2
  exit 1
fi

# --- everything below needs the HISTORY ------------------------------------
# Uniqueness is settled. What follows is containment, which compares HEAD
# against the merge base and therefore genuinely depends on the base ref, the
# merge base, and the base blob. Each of those can be unavailable for reasons
# that are not the author's fault (a shallow clone, a fork checkout without
# the upstream remote, the commit that first adds the changelog), so each
# degrades rather than failing — which is exactly why the uniqueness half must
# NOT live down here (#143). It asks nothing of the history, and gating it
# behind these conditions let a duplicate exit 0 on a message about deletion.

git rev-parse --is-inside-work-tree >/dev/null 2>&1 \
  || skip "not inside a git work tree, so there is no history to compare against"

git rev-parse --verify --quiet "$base_ref^{commit}" >/dev/null \
  || skip "base ref '$base_ref' does not resolve here (a shallow clone, or a fork checkout without the upstream remote)"

merge_base="$(git merge-base "$base_ref" HEAD 2>/dev/null || true)"
[ -n "$merge_base" ] \
  || skip "no merge base between '$base_ref' and HEAD (unrelated histories, or a clone too shallow to reach one)"

# The changelog may not exist at the merge base at all (the commit that adds
# it). Nothing to have deleted, so nothing to assert.
base_file="$(git show "$merge_base:$changelog" 2>/dev/null || true)"
[ -n "$base_file" ] || {
  echo "changelog-monotonic: $changelog does not exist at the merge base ($(git rev-parse --short "$merge_base")) — nothing could have been deleted (uniqueness on HEAD already passed)."
  exit 0
}

base_headings="$(printf '%s\n' "$base_file" | headings)"
head_headings="$(headings < "$changelog")"

# comm -23: lines in the base set that are NOT in the head set — exactly the
# headings this branch removed.
missing="$(comm -23 <(printf '%s\n' "$base_headings") <(printf '%s\n' "$head_headings"))"

if [ -n "$missing" ]; then
  {
    echo "changelog-monotonic: this branch DELETES release heading(s) from $changelog:"
    echo
    printf '%s\n' "$missing" | sed 's/^/    ## /'
    echo
    cat <<EOF
  Present at the merge base ($(git rev-parse --short "$merge_base")), absent on HEAD.

  Release headings are APPEND-ONLY. The ceremony adds one (#96); nothing ever
  legitimately removes one. So this is not a judgement call — it is a defect,
  and almost always the same one (#122, caught in review of #118): an entry
  written under '## Unreleased' REPLACED the heading below it instead of being
  inserted ABOVE it. The shipped section's body is now sitting under
  '## Unreleased', and the version it belonged to has no section at all.

  Nothing else will say so. git merges that edit cleanly — no conflict, no
  signal — and changelog-armed.sh stays green, because the TOP section is
  still the right one for this VERSION. The damage surfaces at the NEXT
  release, when release-notes.sh cannot find the section it extracts by
  heading, or worse, republishes the absorbed prose as if it were new.

  The fix is to put the heading back and INSERT above it, never over it:

      ## Unreleased

      ### Fixed

      - **Your entry**

      ## $(printf '%s\n' "$missing" | head -1) — DATE     <- untouched, still here

  If you are genuinely renaming a released version, that is a rewrite of
  history this guard is meant to stop; say so in the PR and change the guard
  deliberately, in its own commit.
EOF
  } >&2
  exit 1
fi

count="$(printf '%s\n' "$base_headings" | grep -c . || true)"
head_count="$(printf '%s\n' "$head_headings" | grep -c . || true)"

# The success line has two honest forms, because this step now runs on two
# shapes of event. On a push to main the merge base IS HEAD: containment
# compared the file against itself and asserted nothing, and deletion is
# undetectable on that event by construction. Reporting "all N still present"
# there would be the same dishonesty the skip messages were fixed for — a log
# claiming a check that did no work. Uniqueness is the half that actually ran,
# so that is the half the line names.
if [ "$merge_base" = "$(git rev-parse HEAD)" ]; then
  echo "changelog-monotonic: containment vacuous (the merge base IS HEAD, so nothing could have been deleted between them) — uniqueness on HEAD checked $head_count release heading(s)."
else
  echo "changelog-monotonic: all $count release heading(s) at the merge base ($(git rev-parse --short "$merge_base")) are still present in $changelog"
fi