patterns: Added pattern for ID3 tags (#48)

* Add naive first implementation of ID3 patterns

* Refine string handling a bit

* Optimize structs using the static keyword

* Add MIME type pragma and update README

patterns/id3.hexpat

@@ -0,0 +1,156 @@
+#pragma MIME audio/mpeg
+
+#include <std/mem.pat>
+
+namespace v1 {
+
+    struct Tag {
+        char header[3];
+        char title[30];
+        char artist[30];
+        char album[30];
+        char year[4];
+        char comment[30];
+        u8   zero;
+        u8   track;
+        u8   genre;
+    } [[static]];
+
+}
+
+namespace v2 {
+
+    struct SyncSafeInt {
+        u8 bytes[4];
+    } [[sealed, static, format("v2::ssi_value"), transform("v2::ssi_value")]];
+
+    fn ssi_value(ref SyncSafeInt n) {
+        u32 result = 0;
+        for (u8 i = 0, i < 4, i = i + 1) {
+            u8 byteval = n.bytes[i] & 0x7F;
+            u8 shift = 7 * (4 - i - 1);
+            result = result | (byteval << shift);
+        }
+        return result;
+    };
+
+    struct TagVersion {
+        u8 major;
+        u8 revision;
+    };
+    
+    bitfield TagHeaderFlags {
+        unsynchronized : 1;
+        extended       : 1;
+        experimental   : 1;
+        footer         : 1;
+        padding        : 4;
+    };
+    
+    struct TagHeader {
+        char           identifier[3];
+        TagVersion     version;
+        TagHeaderFlags flags;
+        SyncSafeInt    size;
+    } [[static]];
+    
+    bitfield ExtendedFlag {
+        padding      : 1;
+        update       : 1;
+        crcpresent   : 1;
+        restrictions : 1;
+        padding      : 4;
+    };
+
+    struct TagExtendedHeader {
+        SyncSafeInt  size;
+        u8           nflagbytes;
+        ExtendedFlag flags[nflagbytes];
+        u8           data[size];
+    };
+    
+    struct TagFooter {
+        char           identifier[3];
+        TagVersion     version;
+        TagHeaderFlags flags;
+        SyncSafeInt    size;
+    } [[static]];
+    
+    bitfield FrameFlags {
+        padding               : 1;
+        tagalterpreservation  : 1;
+        filealterpreservation : 1;
+        readonly              : 1;
+        padding               : 5;
+        groupid               : 1;
+        padding               : 2;
+        compressed            : 1;
+        encrypted             : 1;
+        unzynchronized        : 1;
+        datalengthindicator   : 1;
+    };
+
+    enum TextEncoding : u8 {
+        ISO88591 = 0x00,
+        UTF16BOM = 0x01,
+        UTF16BE  = 0x02,
+        UTF8     = 0x03,
+    };
+
+    struct StringData {
+        TextEncoding encoding;
+        if (encoding == TextEncoding::UTF16BOM) {
+            u16    bom;
+            char16 data[(parent.size - 3) / 2];
+        } else if (encoding == TextEncoding::UTF16BE)
+            be char16 data[(parent.size - 1) / 2];
+        else
+            char data[parent.size - 1];
+    };
+
+    // Hack to work around the fact, that chars may not be compared
+    union FrameId {
+        char cdata[4];
+        u8   ndata[4];
+    } [[sealed, static, format("v2::impl::format_frame_id")]];
+
+    namespace impl {
+
+    fn format_frame_id(ref FrameId id) {
+        return id.cdata;
+    };
+
+    }
+    
+    struct Frame {
+        FrameId     id;
+        SyncSafeInt size;
+        FrameFlags  flags;
+        if (id.ndata[0] == 'T')
+            StringData data;
+        else
+            u8      data[size];
+    };
+
+    fn has_next_frame(u32 size) {
+        bool hasnext = $ < size;
+        if (hasnext) {
+            hasnext = std::mem::read_unsigned($, 1) != 0;
+            $ = $ - 1;
+        }
+        return hasnext;
+    };
+
+    struct Tag {
+        TagHeader header;
+        if (header.flags.extended)
+            TagExtendedHeader extendedheader;
+        Frame     frames[while(v2::has_next_frame(header.size))];
+        if (header.flags.footer)
+            TagFooter footer;
+    };
+
+}
+
+v2::Tag tag @ 0;
+v1::Tag oldtag @ std::mem::size() - 128;