add h_with_endianness()

src/parsers/endianness.c

@@ -0,0 +1,72 @@
+#include "parser_internal.h"
+
+
+typedef struct {
+    const HParser *p;
+    char endianness;
+} HParseEndianness;
+
+// helper
+static void switch_bit_order(HInputStream *input)
+{
+    assert(input->bit_offset <= 8);
+
+    if((input->bit_offset % 8) != 0) {
+        // switching bit order in the middle of a byte
+        // we leave bit_offset untouched. this means that something like
+        //     le(bits(5)),le(bits(3))
+        // is equivalent to
+        //     le(bits(5),bits(3)) .
+        // on the other hand,
+        //     le(bits(5)),be(bits(5))
+        // will read the same 5 bits twice and discard the top 3.
+    } else {
+        // flip offset (0 <-> 8)
+        input->bit_offset = 8 - input->bit_offset;
+    }
+}
+
+static HParseResult *parse_endianness(void *env, HParseState *state)
+{
+    HParseEndianness *e = env;
+    HParseResult *res = NULL;
+    char diff = state->input_stream.endianness ^ e->endianness;
+
+    if(!diff) {
+        // all the same, nothing to do
+        res = h_do_parse(e->p, state);
+    } else {
+        if(diff & BIT_BIG_ENDIAN)
+            switch_bit_order(&state->input_stream);
+
+        state->input_stream.endianness ^= diff;
+        res = h_do_parse(e->p, state);
+        state->input_stream.endianness ^= diff;
+
+        if(diff & BIT_BIG_ENDIAN)
+            switch_bit_order(&state->input_stream);
+    }
+
+    return res;
+}
+
+static const HParserVtable endianness_vt = {
+    .parse = parse_endianness,
+    .isValidRegular = h_false,
+    .isValidCF = h_false,
+    .desugar = NULL,
+    .compile_to_rvm = h_not_regular,
+};
+
+HParser* h_with_endianness(char endianness, const HParser *p)
+{
+    return h_with_endianness__m(&system_allocator, endianness, p);
+}
+
+HParser* h_with_endianness__m(HAllocator *mm__, char endianness, const HParser *p)
+{
+    HParseEndianness *env = h_new(HParseEndianness, 1);
+    env->endianness = endianness;
+    env->p = p;
+    return h_new_parser(mm__, &endianness_vt, env);
+}