commit 00a411a728b13a6114881546efa1fa34ef3b9f16
Author: Noratrieb <48135649+Noratrieb@users.noreply.github.com>
Date:   Sun Aug 31 16:28:58 2025 +0200

    init

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9f58a8a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+/target
+*.gz
+*.dot
+*.svg
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..e834973
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "zwergli"
+version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..d654e0e
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "zwergli"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..00e524c
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,262 @@
+struct Bitstream<'a> {
+    data: &'a [u8],
+    pos_bit: usize,
+}
+
+impl Bitstream<'_> {
+    fn read_bits_normal(&mut self, mut len: usize) -> Option<u64> {
+        assert!((len as u32) < u64::BITS);
+
+        let mut result = 0;
+
+        while len > 0 {
+            if self.data.is_empty() {
+                return None;
+            }
+
+            let to_read_from_current_byte = std::cmp::min(len % 8, 8 - self.pos_bit);
+            result <<= to_read_from_current_byte;
+            result |=
+                ((self.data[0] >> self.pos_bit) & ((1 << to_read_from_current_byte) - 1)) as u64;
+            len -= to_read_from_current_byte;
+            self.pos_bit += to_read_from_current_byte;
+
+            if self.pos_bit == 8 {
+                self.data = &self.data[1..];
+                self.pos_bit = 0;
+            }
+        }
+
+        Some(result)
+    }
+}
+
+struct HuffmanTree {
+    nodes: Vec<HuffmanNode>,
+}
+
+#[derive(Debug)]
+enum HuffmanNode {
+    Leaf(u16),
+    Cont { zero: usize, one: usize },
+}
+
+enum HuffmanLookupResult {
+    Done(u16),
+    Incomplete { next_state: usize },
+}
+
+impl HuffmanTree {
+    fn fixed_code_lengths() -> [u8; 288] {
+        let mut codes = [0; 288];
+        codes[0..144].fill(8);
+        codes[144..256].fill(9);
+        codes[256..280].fill(7);
+        codes[280..].fill(8);
+        codes
+    }
+
+    fn from_lengths(lengths: &[u8]) -> Self {
+        const MAX_BITS: usize = 10;
+
+        let mut codes = vec![0; lengths.len()];
+
+        let mut bl_count = [0; MAX_BITS];
+        for l in lengths {
+            bl_count[*l as usize] += 1;
+        }
+
+        let mut next_code = [0; MAX_BITS];
+        let mut code = 0;
+        for bits in 1..MAX_BITS {
+            code = (code + bl_count[bits - 1]) << 1;
+            next_code[bits] = code;
+        }
+
+        for n in 0..lengths.len() {
+            let len = lengths[n] as usize;
+            if len != 0 {
+                codes[n] = next_code[len];
+                next_code[len] += 1;
+            }
+        }
+
+        let mut nodes = vec![HuffmanNode::Cont { zero: 0, one: 0 }];
+        for (i, &l) in lengths.iter().enumerate().filter(|(_, l)| **l != 0) {
+            let code = codes[i];
+            let mut parent_node_idx = 0;
+            for j in 0..l {
+                let bit = (code >> (l - 1 - j)) & 0b1;
+                let next_node_idx = nodes.len();
+                let HuffmanNode::Cont { zero, one } = &mut nodes[parent_node_idx] else {
+                    unreachable!()
+                };
+                let this_ref = if bit == 0 { zero } else { one };
+                // are we done?
+                if j == l - 1 {
+                    *this_ref = next_node_idx;
+                    nodes.push(HuffmanNode::Leaf(i as u16));
+                } else {
+                    // has this parent node been inserted already?
+                    if *this_ref > 0 {
+                        // just walk
+                        parent_node_idx = *this_ref;
+                    } else {
+                        // insert and walk
+                        *this_ref = next_node_idx;
+                        nodes.push(HuffmanNode::Cont { zero: 0, one: 0 });
+                        parent_node_idx = next_node_idx;
+                    }
+                }
+            }
+        }
+
+        Self { nodes }
+    }
+
+    fn lookup_with_state(&self, state: usize, bit: u64) -> HuffmanLookupResult {
+        let HuffmanNode::Cont { zero, one } = self.nodes[state] else {
+            unreachable!("invalid state, should point at continuation node");
+        };
+        let next_state = if bit == 0 { zero } else { one };
+        match self.nodes[next_state] {
+            HuffmanNode::Leaf(leaf) => HuffmanLookupResult::Done(leaf),
+            HuffmanNode::Cont { .. } => HuffmanLookupResult::Incomplete { next_state },
+        }
+    }
+
+    fn to_dot(&self) -> String {
+        use std::fmt::Write;
+
+        let mut out = String::new();
+        let mut inner = |tree: &Self| {
+            writeln!(out, "digraph huffman_tree {{")?;
+
+            let root = &tree.nodes[0];
+
+            fn print(
+                tree: &HuffmanTree,
+                out: &mut String,
+                parent: Option<&str>,
+                choice: &str,
+                this_node: &str,
+                node: &HuffmanNode,
+            ) -> std::fmt::Result {
+                match node {
+                    HuffmanNode::Leaf(number) => {
+                        writeln!(
+                            out,
+                            "{parent} -> {number} [label={choice}]",
+                            parent = parent.unwrap(),
+                        )?;
+                    }
+                    HuffmanNode::Cont { zero, one } => {
+                        if let Some(parent) = parent {
+                            writeln!(out, "{parent} -> {this_node} [label={choice}]")?;
+                        }
+                        print(
+                            tree,
+                            out,
+                            Some(this_node),
+                            "0",
+                            &format!("{this_node}0"),
+                            &tree.nodes[*zero],
+                        )?;
+                        print(
+                            tree,
+                            out,
+                            Some(this_node),
+                            "1",
+                            &format!("{this_node}1"),
+                            &tree.nodes[*one],
+                        )?;
+                    }
+                }
+                Ok(())
+            }
+
+            print(tree, &mut out, None, "", "_", root)?;
+
+            writeln!(out, "}}")
+        };
+        inner(self).unwrap();
+        out
+    }
+}
+
+// https://datatracker.ietf.org/doc/html/rfc1951
+pub fn inflate(data: &[u8], out: &mut Vec<u8>) {
+    std::fs::write(
+        "output.dot",
+        HuffmanTree::from_lengths(&HuffmanTree::fixed_code_lengths()).to_dot(),
+    )
+    .unwrap();
+
+    let mut data = Bitstream { data, pos_bit: 0 };
+
+    loop {
+        let bfinal = data.read_bits_normal(1).unwrap();
+
+        let btype = data.read_bits_normal(2).unwrap();
+
+        assert_eq!(btype, 1, "not a static huffman tree construction");
+
+        let tree = HuffmanTree::from_lengths(&HuffmanTree::fixed_code_lengths());
+
+        let mut node_state = 0;
+        loop {
+            let bit = data.read_bits_normal(1).unwrap();
+            let result = tree.lookup_with_state(node_state, bit);
+            match result {
+                HuffmanLookupResult::Done(value) => {
+                    dbg!(value);
+                    node_state = 0;
+
+                    match value {
+                        0..256 => {
+                            out.push(value as u8);
+                        }
+                        256 => break,
+                        257..286 => {
+                            let length = match value {
+                                257..265 => value - (257 - 3),
+                                _ => todo!("lz77 more"),
+                            };
+                        }
+                        286.. => unreachable!("invalid byte"),
+                    }
+                }
+                HuffmanLookupResult::Incomplete { next_state } => node_state = next_state,
+            }
+        }
+
+        if bfinal == 1 {
+            break;
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::HuffmanTree;
+
+    #[test]
+    fn bitstream() {
+        let bytes = [0b110_010_01_u8, 0b010_111_01];
+        let mut stream = super::Bitstream {
+            data: &bytes,
+            pos_bit: 0,
+        };
+        assert_eq!(stream.read_bits_normal(2).unwrap(), 0b01);
+        assert_eq!(stream.read_bits_normal(3).unwrap(), 0b010);
+        assert_eq!(stream.read_bits_normal(5).unwrap(), 0b11001);
+        assert_eq!(stream.read_bits_normal(3).unwrap(), 0b111);
+        assert_eq!(stream.read_bits_normal(3).unwrap(), 0b010);
+    }
+
+    #[test]
+    fn decode() {
+        let lengths = HuffmanTree::fixed_code_lengths();
+        HuffmanTree::from_lengths(&lengths);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..de35c24
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,32 @@
+use std::ffi::CStr;
+
+fn main() {
+    let gz = std::env::args().nth(1).unwrap();
+    let gz = std::fs::read(gz).unwrap();
+
+    assert_eq!(gz[0], 31, "ID");
+    assert_eq!(gz[1], 139, "ID");
+    assert_eq!(gz[2], 8, "compression method");
+
+    let flg = gz[3];
+
+    assert!(flg == 8 || flg == 0); // only FLG.FNAME
+
+    let mut data_start = 10;
+
+    if flg & 0b1000 != 0 {
+        let fname = CStr::from_bytes_until_nul(&gz[10..]).unwrap();
+        dbg!(fname);
+        data_start += fname.count_bytes() + 1;
+    }
+
+    let blocks = &gz[(data_start)..];
+    let blocks = &blocks[..(blocks.len() - 8)]; // crc32 and isize
+
+    let mut out = Vec::new();
+
+    zwergli::inflate(blocks, &mut out);
+
+    dbg!(&out);
+    dbg!(String::from_utf8(out)).ok();
+}