#!/bin/bash

if [ $# -ne 1 -o "$1" = "-h" -o "$1" = "--help" ]; then
  echo "Usage: $(basename $0) <start commit>"
  echo
  echo "Example: $(basename $0) master"
  echo
  echo "This tool runs a regression test on every single commit since the"
  echo "start commit, up to the current commit. Use it to make sure no"
  echo "single commit introduces a regression. Can also be used to find out"
  echo "which commit introduces the regression. Bisecting is fine for the"
  echo "latter goal as well and may be faster, but also requires more thinking"
  echo "of the user."
  echo
  echo "The tool stops as soon as a regression was detected."
  echo
  echo "IMPORTANT: the command to test for a regression is"
  echo
  echo "  make regressiontest"
  echo
  echo "... so this command must be available."
  echo
  echo "Manual equivalent of this tool would be something like this:"
  echo
  echo "  git checkout <current branch>~20"
  echo "  make regresssiontest"
  echo "  git checkout <current branch>~19"
  echo "  make regresssiontest"
  echo "  git checkout <current branch>~18"
  echo "  make regresssiontest"
  echo "  ..."

  exit 1
fi

BASE_BRANCH=${1}
CURRENT_BRANCH=$(git status | awk '/^On branch/ { print $3; exit; }
                                   /^HEAD detached/ { print $4; exit; }')

PARENTS=$(git log --oneline --ancestry-path \
            ${BASE_BRANCH}..${CURRENT_BRANCH} | wc -l)
let PARENTS1=${PARENTS}+1
echo "${PARENTS1} commits to go."

trap "echo -n \"Current commit is \"
      git log HEAD^..HEAD | head -5 | tail -1" 0

while [ ${PARENTS} -ge 0 ]; do
  echo "Next: ${CURRENT_BRANCH}~${PARENTS}"
  sleep 3

  # master and master~0 are different. Checking out the latter leads to a
  # detached head state. This is confusing, avoid it.
  if [ ${PARENTS} -eq 0 ]; then
    git checkout ${CURRENT_BRANCH}
  else
    # The normal thing.
    git checkout ${CURRENT_BRANCH}~${PARENTS}
  fi

  make regressiontests || exit 1

  let PARENTS=${PARENTS}-1
done

echo "All commits ran regression tests successfully."