Merge pull request #369 from jokva/short-endian-swap-attributes

bswap16 short header words in field_forall

lib/src/segy.c

@@ -687,6 +687,14 @@ static int slicelength( int start, int stop, int step ) {
     return (stop - start - 1) / step + 1;
 }
 
+static int32_t bswap_header_word(int32_t f, int word_size) {
+    if (word_size == 4)
+        return bswap32(f);
+
+    int16_t shortf = f;
+    return bswap16(shortf);
+}
+
 int segy_field_forall( segy_file* fp,
                        int field,
                        int start,
@@ -702,6 +710,10 @@ int segy_field_forall( segy_file* fp,
     err = segy_get_field( header, field, &f );
     if( err != SEGY_OK ) return SEGY_INVALID_ARGS;
 
+    // since segy_get_field didn't fail earlier, it is safe to look up the word
+    // size
+    const int word_size = field_size[field];
+
     int slicelen = slicelength( start, stop, step );
 
     // check *once* that we don't look past the end-of-file
@@ -719,7 +731,7 @@ int segy_field_forall( segy_file* fp,
         for( int i = start; slicelen > 0; i += step, ++buf, --slicelen ) {
             segy_seek( fp, i, trace0, trace_bsize );
             get_field( fp->cur, field_size, field, &f );
-            if( lsb ) f = bswap32(f);
+            if (lsb) f = bswap_header_word(f, word_size);
             *buf = f;
         }
 
@@ -744,7 +756,7 @@ int segy_field_forall( segy_file* fp,
         if( readc != 1 ) return SEGY_FREAD_ERROR;
 
         segy_get_field( header, field, &f );
-        if( lsb ) f = bswap32(f);
+        if (lsb) f = bswap_header_word(f, word_size);
         *buf = f;
     }
 

python/test/segy.py

@@ -1643,6 +1643,17 @@ def test_readtrace_int16():
         assert list(tr[40:19:-5]) == [-888, -2213, 5198, -1170, 0]
         assert list(tr[53:50:-1]) == [-2609, -2625, 681]
 
+def test_attributes_shortword_little_endian():
+    f3msb = 'test-data/f3.sgy'
+    f3lsb = 'test-data/f3-lsb.sgy'
+    word = segyio.su.dt
+    # this test (in particular) is a pretty good candidate for fuzzing
+    with segyio.open(f3msb) as msb:
+        with segyio.open(f3lsb, endian = 'little') as lsb:
+            msba = msb.attributes(word)
+            lsba = lsb.attributes(word)
+            npt.assert_array_equal(msba[:], lsba[:])
+
 
 @tmpfiles('test-data/f3.sgy')
 def test_writetrace_int16(tmpdir):