Rewrite in Rust.

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+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "slab"
+version = "0.4.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f92a496fb766b417c996b9c5e57daf2f7ad3b0bebe1ccfca4856390e3d3bb67"
+dependencies = [
+ "autocfg",
+]
+
+[[package]]
+name = "smallvec"
+version = "1.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c5e1a9a646d36c3599cd173a41282daf47c44583ad367b8e6837255952e5c67"
+
+[[package]]
+name = "syn"
+version = "2.0.66"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c42f3f41a2de00b01c0aaad383c5a45241efc8b2d1eda5661812fda5f3cdcff5"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "system-deps"
+version = "6.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a3e535eb8dded36d55ec13eddacd30dec501792ff23a0b1682c38601b8cf2349"
+dependencies = [
+ "cfg-expr",
+ "heck",
+ "pkg-config",
+ "toml",
+ "version-compare",
+]
+
+[[package]]
+name = "target-lexicon"
+version = "0.12.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1fc403891a21bcfb7c37834ba66a547a8f402146eba7265b5a6d88059c9ff2f"
+
+[[package]]
+name = "thiserror"
+version = "1.0.61"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c546c80d6be4bc6a00c0f01730c08df82eaa7a7a61f11d656526506112cc1709"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.61"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "46c3384250002a6d5af4d114f2845d37b57521033f30d5c3f46c4d70e1197533"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "toml"
+version = "0.8.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4e43f8cc456c9704c851ae29c67e17ef65d2c30017c17a9765b89c382dc8bba"
+dependencies = [
+ "serde",
+ "serde_spanned",
+ "toml_datetime",
+ "toml_edit 0.22.13",
+]
+
+[[package]]
+name = "toml_datetime"
+version = "0.6.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4badfd56924ae69bcc9039335b2e017639ce3f9b001c393c1b2d1ef846ce2cbf"
+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "toml_edit"
+version = "0.21.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a8534fd7f78b5405e860340ad6575217ce99f38d4d5c8f2442cb5ecb50090e1"
+dependencies = [
+ "indexmap",
+ "toml_datetime",
+ "winnow 0.5.40",
+]
+
+[[package]]
+name = "toml_edit"
+version = "0.22.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c127785850e8c20836d49732ae6abfa47616e60bf9d9f57c43c250361a9db96c"
+dependencies = [
+ "indexmap",
+ "serde",
+ "serde_spanned",
+ "toml_datetime",
+ "winnow 0.6.8",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
+
+[[package]]
+name = "version-compare"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "852e951cb7832cb45cb1169900d19760cfa39b82bc0ea9c0e5a14ae88411c98b"
+
+[[package]]
+name = "windows-sys"
+version = "0.52.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "282be5f36a8ce781fad8c8ae18fa3f9beff57ec1b52cb3de0789201425d9a33d"
+dependencies = [
+ "windows-targets",
+]
+
+[[package]]
+name = "windows-targets"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6f0713a46559409d202e70e28227288446bf7841d3211583a4b53e3f6d96e7eb"
+dependencies = [
+ "windows_aarch64_gnullvm",
+ "windows_aarch64_msvc",
+ "windows_i686_gnu",
+ "windows_i686_gnullvm",
+ "windows_i686_msvc",
+ "windows_x86_64_gnu",
+ "windows_x86_64_gnullvm",
+ "windows_x86_64_msvc",
+]
+
+[[package]]
+name = "windows_aarch64_gnullvm"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7088eed71e8b8dda258ecc8bac5fb1153c5cffaf2578fc8ff5d61e23578d3263"
+
+[[package]]
+name = "windows_aarch64_msvc"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9985fd1504e250c615ca5f281c3f7a6da76213ebd5ccc9561496568a2752afb6"
+
+[[package]]
+name = "windows_i686_gnu"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88ba073cf16d5372720ec942a8ccbf61626074c6d4dd2e745299726ce8b89670"
+
+[[package]]
+name = "windows_i686_gnullvm"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "87f4261229030a858f36b459e748ae97545d6f1ec60e5e0d6a3d32e0dc232ee9"
+
+[[package]]
+name = "windows_i686_msvc"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "db3c2bf3d13d5b658be73463284eaf12830ac9a26a90c717b7f771dfe97487bf"
+
+[[package]]
+name = "windows_x86_64_gnu"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4e4246f76bdeff09eb48875a0fd3e2af6aada79d409d33011886d3e1581517d9"
+
+[[package]]
+name = "windows_x86_64_gnullvm"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "852298e482cd67c356ddd9570386e2862b5673c85bd5f88df9ab6802b334c596"
+
+[[package]]
+name = "windows_x86_64_msvc"
+version = "0.52.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bec47e5bfd1bff0eeaf6d8b485cc1074891a197ab4225d504cb7a1ab88b02bf0"
+
+[[package]]
+name = "winnow"
+version = "0.5.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f593a95398737aeed53e489c785df13f3618e41dbcd6718c6addbf1395aa6876"
+dependencies = [
+ "memchr",
+]
+
+[[package]]
+name = "winnow"
+version = "0.6.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c3c52e9c97a68071b23e836c9380edae937f17b9c4667bd021973efc689f618d"
+dependencies = [
+ "memchr",
+]
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..094829c
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "qc"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+async-channel = "2.3.1"
+gdk4 = "0.8.2"
+gtk4 = "0.8.2"
+num-bigint = "0.4.5"
+num-traits = "0.2.19"
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..4579b3b
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,95 @@
+{
+  "nodes": {
+    "naersk": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      },
+      "locked": {
+        "lastModified": 1713520724,
+        "narHash": "sha256-CO8MmVDmqZX2FovL75pu5BvwhW+Vugc7Q6ze7Hj8heI=",
+        "owner": "nix-community",
+        "repo": "naersk",
+        "rev": "c5037590290c6c7dae2e42e7da1e247e54ed2d49",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "ref": "master",
+        "repo": "naersk",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1716715802,
+        "narHash": "sha256-usk0vE7VlxPX8jOavrtpOqphdfqEQpf9lgedlY/r66c=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e2dd4e18cc1c7314e24154331bae07df76eb582f",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      }
+    },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1716715802,
+        "narHash": "sha256-usk0vE7VlxPX8jOavrtpOqphdfqEQpf9lgedlY/r66c=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e2dd4e18cc1c7314e24154331bae07df76eb582f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "naersk": "naersk",
+        "nixpkgs": "nixpkgs_2",
+        "utils": "utils"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
+    "utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1710146030,
+        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..d96ac0b
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,26 @@
+{
+  inputs = {
+    naersk.url = "github:nix-community/naersk/master";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, utils, naersk }:
+    utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        deps = with pkgs; [ pkg-config graphene cairo gtk4 ];
+        naersk-lib = pkgs.callPackage naersk {};
+      in
+      {
+        defaultPackage = naersk-lib.buildPackage {
+          src = ./.;
+          nativeBuildInputs = deps;
+        };
+        devShell = with pkgs; mkShell {
+          buildInputs = [ cargo rustc rustfmt pre-commit rustPackages.clippy ] ++ deps;
+          RUST_SRC_PATH = rustPlatform.rustLibSrc;
+        };
+      }
+    );
+}
diff --git a/src/lexer.rs b/src/lexer.rs
new file mode 100644
index 0000000..49d2d04
--- /dev/null
+++ b/src/lexer.rs
@@ -0,0 +1,74 @@
+pub struct Lexer<'a> {
+    pub buf: &'a str,
+}
+
+impl<'a> Lexer<'a> {
+    fn skip_whitespace(&mut self) {
+        while let Some(b' ') = self.buf.as_bytes().first() {
+            self.buf = &self.buf[1..]
+        }
+    }
+
+    fn parse_num(&mut self) -> Option<&'a str> {
+        let mut i = 0;
+        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
+            i += 1;
+        }
+        if let Some(b'.') = self.buf.as_bytes().get(i) {
+            i += 1;
+        }
+        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
+            i += 1;
+        }
+        let (Some(b'e') | Some(b'E')) = self.buf.as_bytes().get(i) else {
+            let t = &self.buf[..i];
+            self.buf = &self.buf[i..];
+            return Some(t);
+        };
+        i += 1;
+        if let Some(b'-') = self.buf.as_bytes().get(i) {
+            i += 1;
+        }
+        while matches!(self.buf.as_bytes().get(i), Some(&b) if b.is_ascii_digit()) {
+            i += 1;
+        }
+        let t = &self.buf[..i];
+        self.buf = &self.buf[i..];
+        Some(t)
+    }
+}
+
+impl<'a> Iterator for Lexer<'a> {
+    type Item = &'a str;
+
+    fn next(&mut self) -> Option<&'a str> {
+        self.skip_whitespace();
+        let &first_byte = self.buf.as_bytes().first()?;
+        if first_byte.is_ascii_digit() || first_byte == b'.' {
+            return self.parse_num();
+        }
+        match first_byte {
+            b'/' => {
+                if let Some(b'/') = self.buf.as_bytes().get(1) {
+                    self.buf = &self.buf[2..];
+                    return Some("//");
+                }
+                self.buf = &self.buf[1..];
+                return Some("/");
+            }
+            b'+' | b'-' | b'*' | b'^' | b'(' | b')' => {
+                let t = &self.buf[..1];
+                self.buf = &self.buf[1..];
+                return Some(t);
+            }
+            _ => (),
+        }
+        let mut i = 0;
+        while let Some(b'a'..=b'z' | b'A'..=b'Z' | b'0'..=b'9') = self.buf.as_bytes().get(i) {
+            i += 1;
+        }
+        let t = &self.buf[..i];
+        self.buf = &self.buf[i..];
+        Some(t)
+    }
+}
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..1f4291e
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,89 @@
+mod lexer;
+mod num;
+mod parser;
+
+use gdk4::{Key, ModifierType};
+use gtk4::glib::{clone, ExitCode};
+use gtk4::prelude::{
+    ApplicationExt, ApplicationExtManual, EditableExt, GridExt, GtkWindowExt, WidgetExt,
+};
+use gtk4::{glib, Application, ApplicationWindow, Entry, EventControllerKey, Grid};
+use std::sync::{Arc, Mutex};
+
+fn send_mail<T>(m: &Mutex<Option<T>>, msg: T) {
+    let mut guard = m.lock().unwrap();
+    *guard = Some(msg);
+}
+
+fn get_mail<T>(m: &Mutex<Option<T>>) -> Option<T> {
+    let mut guard = m.lock().unwrap();
+    std::mem::take(&mut guard)
+}
+
+fn main() -> ExitCode {
+    let app = Application::builder()
+        .application_id("com.github.rhogenson.qc")
+        .build();
+    app.connect_activate(|app| {
+        let window = ApplicationWindow::builder()
+            .application(app)
+            .decorated(false)
+            .resizable(false)
+            .build();
+
+        let grid = Grid::new();
+        window.set_child(Some(&grid));
+
+        let input = Entry::new();
+        grid.attach(&input, 0, 0, 1, 1);
+
+        let output = Entry::builder().editable(false).build();
+        grid.attach(&output, 0, 1, 1, 1);
+
+        let (ping_send, ping_recv) = std::sync::mpsc::sync_channel(1);
+        let mailbox = Arc::new(Mutex::new(None));
+
+        let key_controller = EventControllerKey::new();
+        key_controller.connect_key_released(
+            clone!(@weak window, @weak input, @strong mailbox => move |_, k, _, modifiers| {
+                if k == Key::Escape
+                    || k == Key::Return
+                    || k == Key::bracketleft && modifiers.contains(ModifierType::CONTROL_MASK)
+                {
+                    window.destroy();
+                }
+                send_mail(mailbox.as_ref(), format!("{}", input.text()));
+                let _ = ping_send.try_send(());
+            }),
+        );
+        window.add_controller(key_controller);
+
+        let (output_send, output_recv) = async_channel::bounded(1);
+        let (confirm_send, confirm_recv) = async_channel::bounded(1);
+
+        gtk4::gio::spawn_blocking(move || loop {
+            let Ok(_) = ping_recv.recv() else {
+                return;
+            };
+            let Some(input) = get_mail(mailbox.as_ref()) else {
+                continue;
+            };
+            let Some(result) = parser::parse(&input) else {
+                continue;
+            };
+            output_send.send_blocking(result).unwrap();
+            confirm_recv.recv_blocking().unwrap();
+        });
+
+        glib::spawn_future_local(clone!(@weak output => async move {
+            while let Ok(result) = output_recv.recv().await {
+                output.set_text(&format!("{}", result));
+                confirm_send.send(()).await.unwrap();
+            }
+        }));
+
+        window.present();
+    });
+
+    app.run()
+}
diff --git a/src/num.rs b/src/num.rs
new file mode 100644
index 0000000..f17c076
--- /dev/null
+++ b/src/num.rs
@@ -0,0 +1,183 @@
+use num_bigint::BigInt;
+use num_traits::cast::ToPrimitive;
+use num_traits::pow::Pow;
+use num_traits::FromPrimitive;
+use std::fmt::{Display, Formatter};
+
+pub enum Num {
+    Int(BigInt),
+    Float(f64),
+}
+
+pub fn int(i: BigInt) -> Num {
+    Num::Int(i)
+}
+
+pub fn float(f: f64) -> Num {
+    Num::Float(f)
+}
+
+fn float_from_int(i: &BigInt) -> f64 {
+    if let Some(f) = i.to_f64() {
+        return f;
+    }
+    if i > &BigInt::ZERO {
+        std::f64::INFINITY
+    } else {
+        std::f64::NEG_INFINITY
+    }
+}
+
+impl Num {
+    fn as_float(&self) -> f64 {
+        match self {
+            Num::Int(i) => float_from_int(i),
+            Num::Float(f) => *f,
+        }
+    }
+
+    pub fn pow(self, other: Num) -> Num {
+        match (self, other) {
+            (Num::Int(i1), Num::Int(i2)) => {
+                if i2 < BigInt::ZERO {
+                    return float(float_from_int(&i1).powf(float_from_int(&i2)));
+                }
+                let (_, u2) = i2.into_parts();
+                int(i1.pow(u2))
+            }
+            (n1, n2) => float(n1.as_float().powf(n2.as_float())),
+        }
+    }
+
+    pub fn mul(self, other: Num) -> Num {
+        match (self, other) {
+            (Num::Int(i1), Num::Int(i2)) => int(i1 * i2),
+            (n1, n2) => float(n1.as_float() * n2.as_float()),
+        }
+    }
+
+    pub fn div(self, other: Num) -> Num {
+        float(self.as_float() / other.as_float())
+    }
+
+    pub fn int_div(self, other: Num) -> Num {
+        match (self, other) {
+            (Num::Int(i1), Num::Int(i2)) => int(i1 / i2),
+            (Num::Float(f1), Num::Float(f2)) => {
+                let r = f1 / f2;
+                if let Some(i) = BigInt::from_f64(r) {
+                    return int(i);
+                }
+                float(r)
+            }
+            (Num::Int(i), Num::Float(f)) => {
+                if let Some(fi) = BigInt::from_f64(f) {
+                    return int(i / fi);
+                }
+                float(float_from_int(&i) / f)
+            }
+            (Num::Float(f), Num::Int(i)) => {
+                if let Some(fi) = BigInt::from_f64(f) {
+                    return int(fi / i);
+                }
+                float(f / float_from_int(&i))
+            }
+        }
+    }
+
+    pub fn add(self, other: Num) -> Num {
+        match (self, other) {
+            (Num::Int(i1), Num::Int(i2)) => int(i1 + i2),
+            (n1, n2) => float(n1.as_float() + n2.as_float()),
+        }
+    }
+
+    pub fn sub(self, other: Num) -> Num {
+        match (self, other) {
+            (Num::Int(i1), Num::Int(i2)) => int(i1 - i2),
+            (n1, n2) => Num::Float(n1.as_float() - n2.as_float()),
+        }
+    }
+
+    pub fn sin(self) -> Num {
+        float(self.as_float().sin())
+    }
+
+    pub fn cos(self) -> Num {
+        float(self.as_float().cos())
+    }
+
+    pub fn tan(self) -> Num {
+        float(self.as_float().tan())
+    }
+
+    pub fn sqrt(self) -> Num {
+        match self {
+            Num::Int(i) if i >= BigInt::ZERO => {
+                let s = i.sqrt();
+                if i != &s * &s {
+                    return float(float_from_int(&i).sqrt());
+                }
+                int(s)
+            }
+            n => float(n.as_float().sqrt()),
+        }
+    }
+
+    pub fn log(self) -> Num {
+        float(self.as_float().ln())
+    }
+
+    pub fn floor(self) -> Num {
+        match self {
+            Num::Int(i) => int(i),
+            Num::Float(f) => {
+                if let Some(i) = BigInt::from_f64(f.floor()) {
+                    return int(i);
+                }
+                float(f)
+            }
+        }
+    }
+
+    pub fn ceil(self) -> Num {
+        match self {
+            Num::Int(i) => int(i),
+            Num::Float(f) => {
+                if let Some(i) = BigInt::from_f64(f.ceil()) {
+                    return int(i);
+                }
+                float(f)
+            }
+        }
+    }
+
+    pub fn round(self) -> Num {
+        match self {
+            Num::Int(i) => int(i),
+            Num::Float(f) => {
+                if let Some(i) = BigInt::from_f64(f.round_ties_even()) {
+                    return int(i);
+                }
+                float(f)
+            }
+        }
+    }
+}
+
+fn format_float(f: f64) -> String {
+    let mut s = format!("{f:.7}");
+    while let Some(b'0') = s.as_bytes().get(s.len() - 1) {
+        s.truncate(s.len() - 1);
+    }
+    s
+}
+
+impl Display for Num {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+        match self {
+            Num::Int(n) => write!(f, "{n}"),
+            Num::Float(n) => write!(f, "{}", format_float(*n)),
+        }
+    }
+}
diff --git a/src/parser.rs b/src/parser.rs
new file mode 100644
index 0000000..fda744d
--- /dev/null
+++ b/src/parser.rs
@@ -0,0 +1,202 @@
+use crate::lexer::Lexer;
+use crate::num;
+use crate::num::Num;
+use num_bigint::BigInt;
+use std::str::FromStr;
+
+struct Parser<'a> {
+    r: Lexer<'a>,
+    first_token: Option<&'a str>,
+}
+
+impl<'a> Parser<'a> {
+    fn peek(&mut self) -> Option<&'a str> {
+        if let Some(t) = self.first_token {
+            return Some(t);
+        }
+        self.first_token = self.r.next();
+        self.first_token
+    }
+
+    fn next(&mut self) -> Option<&'a str> {
+        if let Some(t) = std::mem::take(&mut self.first_token) {
+            return Some(t);
+        }
+        self.r.next()
+    }
+
+    fn symbol(&mut self, s: &str) -> bool {
+        let Some(t) = self.peek() else {
+            return false;
+        };
+        if t != s {
+            return false;
+        }
+        self.next();
+        true
+    }
+
+    fn parse_num(&mut self) -> Option<Num> {
+        let negative = self.symbol("-");
+        let t = self.next()?;
+        if t.contains('.') || t.contains('e') || t.contains('E') {
+            let f = f64::from_str(t).ok()?;
+            if negative {
+                return Some(num::float(-f));
+            }
+            return Some(num::float(f));
+        }
+        let i = BigInt::from_str(t).ok()?;
+        if negative {
+            return Some(num::int(-i));
+        }
+        Some(num::int(i))
+    }
+
+    fn paren_expr(&mut self) -> Option<Num> {
+        if !self.symbol("(") {
+            return None;
+        }
+        let n = self.expr()?;
+        if !self.symbol(")") {
+            return None;
+        }
+        Some(n)
+    }
+
+    fn parse_const(&mut self) -> Option<Num> {
+        if self.symbol("e") {
+            return Some(num::float(std::f64::consts::E));
+        }
+        if self.symbol("pi") {
+            return Some(num::float(std::f64::consts::PI));
+        }
+        None
+    }
+
+    fn atom(&mut self) -> Option<Num> {
+        if let Some(n) = self.paren_expr() {
+            return Some(n);
+        }
+        if let Some(n) = self.parse_const() {
+            return Some(n);
+        }
+        self.parse_num()
+    }
+
+    fn parse_fun(&mut self) -> Option<Num> {
+        if self.symbol("sin") {
+            return Some(self.atom()?.sin());
+        }
+        if self.symbol("cos") {
+            return Some(self.atom()?.cos());
+        }
+        if self.symbol("tan") {
+            return Some(self.atom()?.tan());
+        }
+        if self.symbol("sqrt") {
+            return Some(self.atom()?.sqrt());
+        }
+        if self.symbol("log") || self.symbol("ln") {
+            return Some(self.atom()?.log());
+        }
+        if self.symbol("floor") {
+            return Some(self.atom()?.floor());
+        }
+        if self.symbol("ceil") || self.symbol("ceiling") {
+            return Some(self.atom()?.ceil());
+        }
+        if self.symbol("round") {
+            return Some(self.atom()?.round());
+        }
+        None
+    }
+
+    fn app_expr_fold(&mut self, e1: Num) -> Option<Num> {
+        if let Some(e2) = self.paren_expr() {
+            return self.app_expr_fold(e1.mul(e2));
+        }
+        if let Some(e2) = self.parse_const() {
+            return self.app_expr_fold(e1.mul(e2));
+        }
+        if let Some(e2) = self.parse_fun() {
+            return Some(e1.mul(e2));
+        }
+        Some(e1)
+    }
+
+    fn app_expr(&mut self) -> Option<Num> {
+        if let Some(e) = self.parse_fun() {
+            return Some(e);
+        }
+        let e1 = self.atom()?;
+        self.app_expr_fold(e1)
+    }
+
+    fn expt_expr_fold(&mut self, e1: Num) -> Option<Num> {
+        if !self.symbol("^") {
+            return Some(e1);
+        }
+        let e2 = self.app_expr()?;
+        if let Some(e2) = self.expt_expr_fold(e2) {
+            return Some(e1.pow(e2));
+        }
+        None
+    }
+
+    fn expt_expr(&mut self) -> Option<Num> {
+        let n = self.app_expr()?;
+        self.expt_expr_fold(n)
+    }
+
+    fn mul_expr_fold(&mut self, e1: Num) -> Option<Num> {
+        if self.symbol("*") {
+            let e2 = self.expt_expr()?;
+            let e = e1.mul(e2);
+            return self.mul_expr_fold(e);
+        }
+        if self.symbol("/") {
+            let e2 = self.expt_expr()?;
+            let e = e1.div(e2);
+            return self.mul_expr_fold(e);
+        }
+        if self.symbol("//") {
+            let e2 = self.expt_expr()?;
+            let e = e1.int_div(e2);
+            return self.mul_expr_fold(e);
+        }
+        Some(e1)
+    }
+
+    fn mul_expr(&mut self) -> Option<Num> {
+        let n = self.expt_expr()?;
+        self.mul_expr_fold(n)
+    }
+
+    fn add_expr_fold(&mut self, e1: Num) -> Option<Num> {
+        if self.symbol("+") {
+            let e2 = self.mul_expr()?;
+            let e = e1.add(e2);
+            return self.add_expr_fold(e);
+        }
+        if self.symbol("-") {
+            let e2 = self.mul_expr()?;
+            let e = e1.sub(e2);
+            return self.add_expr_fold(e);
+        }
+        Some(e1)
+    }
+
+    fn expr(&mut self) -> Option<Num> {
+        let n = self.mul_expr()?;
+        self.add_expr_fold(n)
+    }
+}
+
+pub fn parse(expr: &str) -> Option<Num> {
+    let mut p = Parser {
+        r: Lexer { buf: expr },
+        first_token: None,
+    };
+    p.expr()
+}