# Test Case: LZ4 Deterministic Random-Offset Reads

**Test ID**: TC087-lz4-random-read

**G5 fixture contract**: Use [G5-MANUAL-SETUP.md](G5-MANUAL-SETUP.md).
Its generated paths, source comparisons, structured corruption offsets, and
cleanup rules supersede placeholder examples in this file.
**Category**: Compression
**Priority**: High

## Objective

Verify non-sequential reads from deterministic offsets in an LZ4-compressed
file and compare every returned byte with the source. `$RANDOM` and timing-only
checks are not evidence of data correctness.

## Fixture and Helper

```sh
work=$(mktemp -d /tmp/repo22-tc087.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
cc -O2 -Wall -Wextra -std=c17 tests/read_probe.c -o "$work/read_probe"
dump.erofs --path=/compressed.bin -e "$work/fixtures/valid-lz4.erofs"
```

## FreeBSD Procedure

```sh
mkdir "$work/mnt"
unit=$(mdconfig -a -t vnode -f "$work/fixtures/valid-lz4.erofs")
mount -t erofs -o ro "/dev/$unit" "$work/mnt"
size=$(stat -f %z "$work/fixtures/source/compressed.bin")
test "$size" -gt 1052672

index=0
for offset in 0 4096 65536 1048576 $((size - 4096)); do
  "$work/read_probe" pread "$work/mnt/compressed.bin" \
    "$offset" 4096 "$work/guest-$index.bin"
  "$work/read_probe" pread "$work/fixtures/source/compressed.bin" \
    "$offset" 4096 "$work/source-$index.bin"
  cmp "$work/source-$index.bin" "$work/guest-$index.bin"
  index=$((index + 1))
done

umount "$work/mnt"
mdconfig -d -u "${unit#md}"
```

## Expected Results

All five fixed reads, including the final 4 KiB of the file, return exactly
4096 bytes and compare equal to their corresponding source ranges. No
performance ordering is asserted.