Add gbdiff.bash script

This script allows diffing two Game Boy ROMs using `xxd` and `diff` and
attempts to augment the diff using information pulled from SYM files.
It's not part of RGBDS proper, but can be useful to debug tests (when ROMs fail
to match).

contrib/gbdiff.bash

@@ -0,0 +1,71 @@
+#!/bin/bash
+
+# SPDX-License-Identifier: MIT
+#
+# Copyright (c) 2020 Eldred Habert
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+STATE=0
+diff <(xxd $1) <(xxd $2) | while read -r LINE; do
+	if [ $STATE -eq 0 ]; then
+		# Discard first line (line info)
+		STATE=1
+	elif [ "$LINE" = '---' ]; then
+		# Separator between files switches states
+		echo $LINE
+		STATE=3
+	elif echo $LINE | grep -Eq '^[0-9]+(,[0-9]+)?c[0-9]+(,[0-9]+)?'; then
+		# Line info resets the whole thing
+		STATE=1
+	elif [ $STATE -eq 1  -o  $STATE -eq 3 ]; then
+		# Compute the GB address from the ROM offset
+		OFS=$(echo $LINE | cut -d ' ' -f 2 | tr -d ':')
+		BANK=$((0x$OFS / 0x4000))
+		ADDR=$((0x$OFS % 0x4000 + ($BANK != 0) * 0x4000))
+		# Try finding the preceding symbol closest to the diff
+		if [ $STATE -eq 1 ]; then
+			STATE=2
+			SYMFILE=$(echo $1 | cut -d '.' -f 1).sym
+		else
+			STATE=4
+			SYMFILE=$(echo $2 | cut -d '.' -f 1).sym
+		fi
+		EXTRA=$(if [ -f "$SYMFILE" ]; then
+			# Read the sym file for such a symbol
+			# Ignore comment lines, only pick matching bank
+			grep -Fv ';' "$SYMFILE" |
+			 grep -Ei $(printf "^%02x:" $BANK) |
+			 while read -r SYMADDR SYM; do
+				SYMADDR=$((0x$(echo $SYMADDR | cut -d ':' -f 2)))
+				if [ $SYMADDR -le $ADDR ]; then
+					printf " (%s+%#x)\n" $SYM $(($ADDR - $SYMADDR))
+				fi
+			# TODO: assumes sorted sym files
+			done | tail -n 1
+		fi)
+		printf "%02x:%04x %s\n" $BANK $ADDR $EXTRA
+	fi
+	if [ $STATE -eq 2  -o  $STATE -eq 4 ]; then
+		OFS=$(echo $LINE | cut -d ' ' -f 2 | tr -d ':')
+		BANK=$((0x$OFS / 0x4000))
+		ADDR=$((0x$OFS % 0x4000 + ($BANK != 0) * 0x4000))
+		printf "%s %02x:%04x: %s\n" "$(echo $LINE | cut -d ' ' -f 1)" $BANK $ADDR "$(echo $LINE | cut -d ' ' -f 3-)"
+	fi
+done