liblzma: Allow nice_len 2 and 3 even if match finder requires 3 or 4.

That is, if the specified nice_len is smaller than the minimum
of the match finder, silently use the match finder's minimum value
instead of reporting an error. The old behavior is annoying to users
and it complicates xz options handling too.

2 files changed, 18 insertions, 5 deletions

diff --git a/src/liblzma/lz/lz_encoder.c b/src/liblzma/lz/lz_encoder.c
index 2dad7e5f..702582ce 100644
--- a/src/liblzma/lz/lz_encoder.c
+++ b/src/liblzma/lz/lz_encoder.c
@@ -293,11 +293,15 @@ lz_encoder_prepare(lzma_mf *mf, const lzma_allocator *allocator,
 		return true;
 	}
 
-	// Calculate the sizes of mf->hash and mf->son and check that
-	// nice_len is big enough for the selected match finder.
-	const uint32_t hash_bytes = lz_options->match_finder & 0x0F;
-	if (hash_bytes > mf->nice_len)
-		return true;
+	// Calculate the sizes of mf->hash and mf->son.
+	//
+	// NOTE: Since 5.3.5beta the LZMA encoder ensures that nice_len
+	// is big enough for the selected match finder. This makes it
+	// easier for applications as nice_len = 2 will always be accepted
+	// even though the effective value can be slightly bigger.
+	const uint32_t hash_bytes
+			= mf_get_hash_bytes(lz_options->match_finder);
+	assert(hash_bytes <= mf->nice_len);
 
 	const bool is_bt = (lz_options->match_finder & 0x10) != 0;
 	uint32_t hs;
diff --git a/src/liblzma/lz/lz_encoder.h b/src/liblzma/lz/lz_encoder.h
index e249beba..41439408 100644
--- a/src/liblzma/lz/lz_encoder.h
+++ b/src/liblzma/lz/lz_encoder.h
@@ -220,6 +220,15 @@ typedef struct {
 // are called `read ahead'.
 
 
+/// Get how many bytes the match finder hashes in its initial step.
+/// This is also the minimum nice_len value with the match finder.
+static inline uint32_t
+mf_get_hash_bytes(lzma_match_finder match_finder)
+{
+	return (uint32_t)match_finder & 0x0F;
+}
+
+
 /// Get pointer to the first byte not ran through the match finder
 static inline const uint8_t *
 mf_ptr(const lzma_mf *mf)