Fix Stepmania/ITG chart BGCHANGES videos/images

fix_stepmania_chart_links.sh

#! /usr/bin/env bash
# Fix simple .SM and .SSC chart mistakes when the first entry of #BGCHANGES
# points to a file that doesn't exist and when there's another file of the 
# same extension in the song directory.
#
# E.g. Ben Speirs' SPEIRMIX GALAXY has a lot of these mismatching filenames.

find -type f \
  \( -iname '*.sm' -or  -iname '*.ssc' \) \
  -exec grep -l '#BGCHANGES:[^;]' {} + |
while read -r chart; do
    dir=$(dirname "$chart")

    orig_bg=$(grep -o '#BGCHANGES:[-0-9.]*=[^=]*' "$chart")
    avi=$(echo "$orig_bg" | cut -d= -f2)
    ext=$(echo "$avi" | sed 's/.*\.//')

    [[ -z $orig_bg ]] && { echo "NO BG: $chart"; break; }
    [[ -z $avi ]] && { echo "NO AVI: $chart / $orig_bg"; break; }

    cur_file=$dir/$avi
    [[ -f $cur_file ]] && continue

    globbed=$(echo "$dir"/*."$ext")
    [[ -f $globbed ]] || { echo "No matching file extension: $chart / $globbed"; continue; }

    new_file=$(basename "$globbed")
    if grep -q '[,=]' <<< "$new_file"; then
        echo "INVALID FILENAME (',' and '=' forbidden): $dir / $cur_file / $globbed"
        continue
    fi

    [[ $globbed != $cur_file ]] || continue

    new_bg="$(echo "$orig_bg" | cut -d= -f1)=$new_file"
    replace_bg() {
        python -c '
import sys;_,old,new=sys.argv
for line in sys.stdin:print(line.replace(old,new))
        ' "$orig_bg" "$new_bg" < "$1"
    }

    # Sanity checks:
    new_new_res=$(replace_bg "$chart" | grep '#BGCHANGES:')
    new_new_bg=$(echo "$new_new_res" | grep -o '#BGCHANGES:[-0-9.]*=[^=]*')
    new_avi=$(echo "$new_new_bg" | cut -d= -f2)

    [[ $new_avi == $avi ]] && {
        echo "Substitution failure! $chart"
        echo $globbed != $cur_file
        break
    }

    [[ -f $dir/$new_avi ]] || {
        echo "FATAL: New file somehow doesn't work!"
        echo "old: $dir/$avi"
        echo "new: $dir/$new_avi"
        echo "was supposed to be: $globbed"
        break
    }

    replace_bg "$chart" > "$chart.new"
    cmp "$chart" "$chart.new" || { echo "Chart didn't change: $chart.new"; break; }
    cp -a "$chart" "$chart.bak"
    mv "$chart.new" "$chart"
done