Initial working version (but lots of temp stuff)

src/header.rs

@@ -0,0 +1,53 @@
+use crate::consts::{QOI_HEADER_SIZE, QOI_MAGIC};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct Header {
+    pub magic: [u8; 4],
+    pub width: u32,
+    pub height: u32,
+    pub channels: u8,
+    pub colorspace: u8,
+}
+
+impl Default for Header {
+    fn default() -> Self {
+        Self { magic: QOI_MAGIC, width: 0, height: 0, channels: 3, colorspace: 0 }
+    }
+}
+
+const fn u32_to_be(v: u32) -> [u8; 4] {
+    [
+        ((0xff000000 & v) >> 24) as u8,
+        ((0xff0000 & v) >> 16) as u8,
+        ((0xff00 & v) >> 8) as u8,
+        (0xff & v) as u8,
+    ]
+}
+
+const fn u32_from_be(v: &[u8]) -> u32 {
+    ((v[0] as u32) << 24) | ((v[1] as u32) << 16) | ((v[2] as u32) << 8) | (v[3] as u32)
+}
+
+impl Header {
+    pub const SIZE: usize = QOI_HEADER_SIZE;
+
+    pub(crate) fn to_bytes(&self) -> [u8; QOI_HEADER_SIZE] {
+        let mut out = [0; QOI_HEADER_SIZE];
+        out[..4].copy_from_slice(&self.magic);
+        out[4..8].copy_from_slice(&u32_to_be(self.width));
+        out[8..12].copy_from_slice(&u32_to_be(self.height));
+        out[12] = self.channels;
+        out[13] = self.colorspace;
+        out
+    }
+
+    pub(crate) fn from_bytes(v: [u8; QOI_HEADER_SIZE]) -> Self {
+        let mut out = Self::default();
+        out.magic.copy_from_slice(&v[..4]);
+        out.width = u32_from_be(&v[4..8]);
+        out.height = u32_from_be(&v[8..12]);
+        out.channels = v[12];
+        out.colorspace = v[13];
+        out
+    }
+}