Add header reading

src/chunks/chunks.odin

@@ -2,6 +2,7 @@ package chunks
 
 import "base:runtime"
 import "core:fmt"
+import "core:io"
 import "core:os"
 import "core:strconv"
 import "core:strings"
@@ -42,6 +43,12 @@ Writer :: struct {
 	asset_table: AssetTable,
 }
 
+Error :: union #shared_nil {
+	io.Error,
+	os.Error,
+	Read_Error,
+}
+
 // -----------------------------------------
 
 writer_init :: proc(

src/chunks/read_metadata.odin

@@ -1 +1,69 @@
 package chunks
+
+import "base:runtime"
+import "core:io"
+import "core:mem"
+import "core:os"
+
+// -----------------------------------------
+
+Reader :: struct {}
+
+Read_Error :: enum u8 {
+	None             = 0,
+	File_Not_A_Chunk = 1,
+	Unimplemented    = 127,
+	Okay             = None,
+}
+
+// -----------------------------------------
+
+// read header
+// file_exists() func
+// read_file() -> check chunks -> check fs -> fail
+
+read_header :: proc(
+	chunk1: ^os.File,
+	allocator := context.allocator,
+) -> (
+	Header,
+	Error,
+) {
+	return read_header_impl(chunk1, allocator)
+}
+
+read_header_by_path :: proc(
+	chunk1: string,
+	allocator := context.allocator,
+) -> (
+	header: Header,
+	err: Error,
+) {
+	chunk1_file := os.open(chunk1, {.Read}) or_return
+
+	return read_header_impl(chunk1_file, allocator)
+}
+
+@(private)
+read_header_impl :: proc(
+	chunk1: ^os.File,
+	allocator: runtime.Allocator,
+) -> (
+	header: Header,
+	err: Error,
+) {
+	chunk1_stream := os.to_stream(chunk1)
+
+	header_bytes := mem.ptr_to_bytes(&header)
+	io.read_at(chunk1_stream, header_bytes, 0) or_return
+
+	if header.magic_string != MAGIC_STRING || header.version != 1 {
+		return {}, .File_Not_A_Chunk
+	}
+
+	if header.compression != 0 {
+		return {}, .Unimplemented
+	}
+
+	return header, nil
+}