Policy and contribution

src/descriptor/checksum.rs

@@ -0,0 +1,64 @@
+use std::iter::FromIterator;
+
+use crate::descriptor::Error;
+
+const INPUT_CHARSET: &str =  "0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
+const CHECKSUM_CHARSET: &str = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+
+fn poly_mod(mut c: u64, val: u64) -> u64 {
+    let c0 = c >> 35;
+    c = ((c & 0x7ffffffff) << 5) ^ val;
+    if c0 & 1 > 0 {
+        c ^= 0xf5dee51989
+    };
+    if c0 & 2 > 0 {
+        c ^= 0xa9fdca3312
+    };
+    if c0 & 4 > 0 {
+        c ^= 0x1bab10e32d
+    };
+    if c0 & 8 > 0 {
+        c ^= 0x3706b1677a
+    };
+    if c0 & 16 > 0 {
+        c ^= 0x644d626ffd
+    };
+
+    c
+}
+
+pub fn get_checksum(desc: &str) -> Result<String, Error> {
+    let mut c = 1;
+    let mut cls = 0;
+    let mut clscount = 0;
+    for ch in desc.chars() {
+        let pos = INPUT_CHARSET
+            .find(ch)
+            .ok_or(Error::InvalidDescriptorCharacter(ch))? as u64;
+        c = poly_mod(c, pos & 31);
+        cls = cls * 3 + (pos >> 5);
+        clscount += 1;
+        if clscount == 3 {
+            c = poly_mod(c, cls);
+            cls = 0;
+            clscount = 0;
+        }
+    }
+    if clscount > 0 {
+        c = poly_mod(c, cls);
+    }
+    (0..8).for_each(|_| c = poly_mod(c, 0));
+    c ^= 1;
+
+    let mut chars = Vec::with_capacity(8);
+    for j in 0..8 {
+        chars.push(
+            CHECKSUM_CHARSET
+                .chars()
+                .nth(((c >> (5 * (7 - j))) & 31) as usize)
+                .unwrap(),
+        );
+    }
+
+    Ok(String::from_iter(chars))
+}