From 95fc6832fd8b97257640731b24efacf4828fb0c0 Mon Sep 17 00:00:00 2001
From: Kai Schmidt <kaikaliischmidt@gmail.com>
Date: Fri, 29 Nov 2024 12:46:12 -0800
Subject: [PATCH] add epsilon constant

---
 changelog.md          |  2 ++
 src/lex.rs            | 46 +++++++++++++++++++++++++++++++++++++++----
 src/primitive/defs.rs |  4 ++++
 3 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/changelog.md b/changelog.md
index 3ab823f97..0895b088d 100644
--- a/changelog.md
+++ b/changelog.md
@@ -30,6 +30,8 @@ This version is not yet released. If you are reading this on the website, then t
 - Stabilize [`last ⊣`](https://uiua.org/docs/last)
   - Getting the last row is a very common operation
 - Negative indices to [`pick ⊡`](https://uiua.org/docs/pick) and [`select ⊏`](https://uiua.org/docs/select) now always use a fill value if available
+- Add an `ε` shadowable constant for the machine epsilon
+  - You can type it as `Epsilon`
 - Add [`&ep`](https://uiua.org/docs/&ep) and [`&epf`](https://uiua.org/docs/&epf) system functions for easier printing to stderr
 - Change [`backward 𝄈`](https://uiua.org/docs/backward)'s glyph to `𝄈`. Code using `˜` will continue to work and will be formatted as `𝄈`.
 - Add the experimental [`or ∨`](https://uiua.org/docs/or) function
diff --git a/src/lex.rs b/src/lex.rs
index 40be0da2f..354908865 100644
--- a/src/lex.rs
+++ b/src/lex.rs
@@ -1,7 +1,7 @@
 //! The Uiua lexer
 
 use std::{
-    collections::VecDeque,
+    collections::{BTreeMap, VecDeque},
     error::Error,
     fmt,
     hash::Hash,
@@ -1615,7 +1615,7 @@ pub fn is_custom_glyph(c: &str) -> bool {
 }
 
 pub(crate) fn canonicalize_ident(ident: &str) -> Ident {
-    canonicalize_subscripts(&canonicalize_exclams(ident))
+    canonicalize_special(canonicalize_subscripts(canonicalize_exclams(ident)))
 }
 
 /// Rewrite the identifier with the same number of exclamation points
@@ -1640,9 +1640,9 @@ fn place_exclams(ident: &str, count: usize) -> Ident {
 }
 
 /// Rewrite the identifier with numerals preceded by `__` replaced with subscript characters
-fn canonicalize_subscripts(ident: &str) -> Ident {
+fn canonicalize_subscripts(ident: Ident) -> Ident {
     if !ident.contains('_') {
-        return ident.into();
+        return ident;
     }
     // This hasty canonicalization is okay because the stricter
     // rules about the syntax are handled in the lexer
@@ -1657,6 +1657,44 @@ fn canonicalize_subscripts(ident: &str) -> Ident {
         .collect()
 }
 
+thread_local! {
+    static SPECIAL: BTreeMap<&'static str, &'static str> = [
+        ("Alpha", "α"),
+        ("Beta", "β"),
+        ("Gamma", "γ"),
+        ("Delta", "δ"),
+        ("Epsilon", "ε"),
+        ("Zeta", "ζ"),
+        ("Iota", "ι"),
+        ("Kappa", "κ"),
+        ("Lambda", "λ"),
+        ("Mu", "μ"),
+        ("Nu", "ν"),
+        ("Xi", "ξ"),
+        ("Omicron", "ο"),
+        ("Rho", "ρ"),
+        ("Sigma", "σ"),
+        ("Upsilon", "υ"),
+        ("Phi", "φ"),
+        ("Chi", "χ"),
+        ("Psi", "ψ"),
+        ("Omega", "ω"),
+    ].into()
+}
+
+fn canonicalize_special(ident: Ident) -> Ident {
+    let end = ident
+        .find(|c: char| "!‼₋".contains(c) || SUBSCRIPT_DIGITS.contains(&c))
+        .unwrap_or(ident.len());
+    if let Some(replacement) = SPECIAL.with(|map| map.get(&ident[..end]).copied()) {
+        let mut new = Ident::from(replacement);
+        new.push_str(&ident[end..]);
+        new
+    } else {
+        ident
+    }
+}
+
 fn pick_subscript(neg: bool, n: Option<i32>, overflow: bool) -> Subscript {
     if overflow {
         return Subscript::TooLarge;
diff --git a/src/primitive/defs.rs b/src/primitive/defs.rs
index fbc5ba09a..ea93b7d90 100644
--- a/src/primitive/defs.rs
+++ b/src/primitive/defs.rs
@@ -154,6 +154,10 @@ constant!(
     ("W", Math, WILDCARD_NAN),
     /// The maximum integer that can be represented exactly
     ("MaxInt", Math, 2f64.powi(53)),
+    /// The machine epsilon for Uiua numbers
+    ///
+    /// It is the difference between 1 and the next larger representable number.
+    ("ε", Math, f64::EPSILON),
     /// A string identifying the operating system
     ("Os", System, std::env::consts::OS),
     /// A string identifying family of the operating system