#!/usr/bin/env /bin/bash
# Co-Authored by: Claude (Sonnet 5.5 Medium)
# Usage: render.sh src/path/file.in [../vars.sh]
set -euo pipefail

in=$1
vars_file=${2:-../vars.sh}

case $in in
    src/*) ;;
    *) echo "render.sh: input must be under src/: $in" >&2; exit 1 ;;
esac

# src/etc/issue.in -> build/etc/issue
out=build/${in#src/}
out=${out%.in}

# Load variables from vars.sh and export them for envsubst
if [[ -n $vars_file ]]; then
    set -a
    # shellcheck disable=SC1090
    source "$vars_file"
    set +a
fi

# Only substitute ${VAR} references that actually appear in the template,
# so anything else (like \n or \l) is left untouched
refs=$(grep -oE '\$\{[A-Za-z_][A-Za-z0-9_]*\}' "$in" | sort -u || true)

# Fail loudly on undefined variables instead of silently emitting empty strings
for ref in $refs; do
    name=${ref#\$\{}; name=${name%\}}
    if [[ -z ${!name+x} ]]; then
        echo "render.sh: $in uses undefined variable $name" >&2
        exit 1
    fi
done

mkdir -p "$(dirname "$out")"

# Write to a temp file first so a failure never leaves a half-written output
tmp=$(mktemp "$out.XXXXXX")
trap 'rm -f "$tmp"' EXIT
envsubst "$(echo $refs)" < "$in" > "$tmp"
chmod 644 "$tmp"
mv "$tmp" "$out"
trap - EXIT