commit 3cf5b5e6522cc867bbb275be4d2a3b8b379a3981
Author: mehbark <terezi@pyrope.net>
Date:   Wed Apr 22 17:56:49 2026 -0400

    initial

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..bdc0075
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/target
+ciphertext.txt
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..1131c83
--- /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 = "substitution-helper"
+version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..406fe44
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "substitution-helper"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..541b5f1
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,103 @@
+use std::{
+    collections::HashMap,
+    env,
+    io::{self, Read, Write},
+    process,
+};
+
+fn main() {
+    let mut subst = HashMap::new();
+    for pair in env::args().skip(1) {
+        let &[from, to] = pair.as_bytes() else {
+            eprintln!("Usage: substitution-helper AB CD EF ...");
+            process::exit(1);
+        };
+        subst.insert(from.to_ascii_uppercase(), to.to_ascii_uppercase());
+    }
+
+    let mut inp = String::new();
+    io::stdin().read_to_string(&mut inp).unwrap();
+    let mut out = io::stdout().lock();
+
+    let mut byte_frequencies: HashMap<[char; 1], usize> = HashMap::new();
+    for c in inp.chars().filter(char::is_ascii_alphabetic) {
+        *byte_frequencies
+            .entry([c.to_ascii_uppercase()])
+            .or_default() += 1;
+    }
+
+    println!("Letters:");
+    print_freqs(byte_frequencies, &subst);
+    println!();
+
+    let mut digram_freqs: HashMap<[char; 2], usize> = HashMap::new();
+    for slice in inp.as_bytes().windows(2) {
+        let [a, b] = slice else {
+            continue;
+        };
+        if !(a.is_ascii_alphabetic() && b.is_ascii_alphabetic()) {
+            continue;
+        }
+        *digram_freqs.entry([*a as char, *b as char]).or_default() += 1;
+    }
+
+    println!("Digrams:");
+    print_freqs(digram_freqs, &subst);
+    println!();
+
+    let mut trigram_freqs: HashMap<[char; 3], usize> = HashMap::new();
+    for slice in inp.as_bytes().windows(3) {
+        let [a, b, c] = slice else {
+            continue;
+        };
+        if !(a.is_ascii_alphabetic() && b.is_ascii_alphabetic() && c.is_ascii_alphabetic()) {
+            continue;
+        }
+        *trigram_freqs
+            .entry([*a as char, *b as char, *c as char])
+            .or_default() += 1;
+    }
+
+    println!("Trigrams:");
+    print_freqs(trigram_freqs, &subst);
+    println!();
+
+    for byte in inp.bytes() {
+        let new_byte = subst
+            .get(&byte.to_ascii_uppercase())
+            .copied()
+            .unwrap_or(byte.to_ascii_lowercase());
+        // .unwrap_or(b'.');
+        out.write_all(&[new_byte]).unwrap();
+    }
+}
+
+fn print_freqs<const N: usize>(freqs: HashMap<[char; N], usize>, subst: &HashMap<u8, u8>) {
+    let total_freqs: usize = freqs.values().sum();
+
+    let mut freqs: Vec<_> = freqs
+        .into_iter()
+        .filter(|(_, n)| *n > 1 && (*n as f64 / total_freqs as f64) > 0.001)
+        .collect();
+    freqs.sort_by_key(|(f, count)| (*count, *f));
+
+    for (c, count) in freqs.iter().rev() {
+        println!(
+            "{:>3}: {:4.1}% ({count:4})",
+            c.iter()
+                .map(|c| {
+                    let Ok(b): Result<u8, _> = (*c).try_into() else {
+                        panic!()
+                    };
+
+                    if let Some(new) = subst.get(&b) {
+                        (*new as char).to_ascii_uppercase()
+                    } else {
+                        (b as char).to_ascii_lowercase()
+                    }
+                })
+                .collect::<String>(),
+            (*count as f32 / total_freqs as f32) * 100.
+        );
+    }
+}