1 files changed, 143 insertions, 0 deletions
diff --git a/src/percent_encoded_character_decoder.rs b/src/percent_encoded_character_decoder.rs
new file mode 100644
index 0000000..2fd2753
--- /dev/null
+++ b/src/percent_encoded_character_decoder.rs
@@ -0,0 +1,143 @@
+#![warn(clippy::pedantic)]
+
+use std::collections::HashSet;
+use std::convert::TryFrom;
+
+// This is the character set containing just numbers.
+lazy_static! {
+    static ref DIGIT: HashSet<char> =
+        ('0'..='9')
+        .collect();
+}
+
+// This is the character set containing just the upper-case
+// letters 'A' through 'F', used in upper-case hexadecimal.
+lazy_static! {
+    static ref HEX_UPPER: HashSet<char> =
+        ('A'..='F')
+        .collect();
+}
+
+// This is the character set containing just the lower-case
+// letters 'a' through 'f', used in lower-case hexadecimal.
+lazy_static! {
+    static ref HEX_LOWER: HashSet<char> =
+        ('a'..='f')
+        .collect();
+}
+
+// TODO: Learn about using thiserror to define library errors
+// [14:05] ABuffSeagull: You should use https://lib.rs/crates/thiserror for the errors
+// [14:07] 715209: i also recommend thiserror
+#[derive(Debug, Clone, PartialEq)]
+pub enum Error {
+    IllegalCharacter,
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Error::IllegalCharacter => {
+                write!(f, "illegal character")
+            },
+        }
+    }
+}
+
+pub struct PercentEncodedCharacterDecoder {
+    decoded_character: u8,
+    digits_left: usize,
+}
+
+impl PercentEncodedCharacterDecoder {
+    pub fn new() -> Self {
+        Self{
+            decoded_character: 0,
+            digits_left: 2,
+        }
+    }
+
+    pub fn next(
+        &mut self,
+        c: char
+    ) -> Result<Option<u8>, Error> {
+        self.shift_in_hex_digit(c)?;
+        self.digits_left -= 1;
+        if self.digits_left == 0 {
+            let output = self.decoded_character;
+            self.reset();
+            Ok(Some(output))
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn reset(&mut self) {
+        self.decoded_character = 0;
+        self.digits_left = 2;
+    }
+
+    fn shift_in_hex_digit(
+        &mut self,
+        c: char
+    ) -> Result<(), Error> {
+        self.decoded_character <<= 4;
+        if let Some(ci) = c.to_digit(16) {
+            self.decoded_character += u8::try_from(ci).unwrap();
+        } else {
+            self.reset();
+            return Err(Error::IllegalCharacter);
+        }
+        Ok(())
+    }
+
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn good_sequences() {
+
+        // TODO: consider named tuples instead
+        //
+        // [14:07] LeinardoSmith: Looks like there is a macro for named tuples:
+        // https://docs.rs/named_tuple/0.1.3/named_tuple/
+        struct TestVector {
+            sequence: [char; 2],
+            expected_output: u8,
+        }
+        let test_vectors = [
+            TestVector{sequence: ['4', '1'], expected_output: b'A'},
+            TestVector{sequence: ['5', 'A'], expected_output: b'Z'},
+            TestVector{sequence: ['6', 'e'], expected_output: b'n'},
+            TestVector{sequence: ['e', '1'], expected_output: b'\xe1'},
+            TestVector{sequence: ['C', 'A'], expected_output: b'\xca'},
+        ];
+        for test_vector in &test_vectors {
+            let mut pec = PercentEncodedCharacterDecoder::new();
+            assert_eq!(
+                Ok(None),
+                pec.next(test_vector.sequence[0])
+            );
+            assert_eq!(
+                Ok(Some(test_vector.expected_output)),
+                pec.next(test_vector.sequence[1])
+            );
+        }
+    }
+
+    #[test]
+    fn bad_sequences() {
+        let test_vectors = [
+            'G', 'g', '.', 'z', '-', ' ', 'V',
+        ];
+        for test_vector in &test_vectors {
+            let mut pec = PercentEncodedCharacterDecoder::new();
+            assert!(pec.next(*test_vector).is_err());
+        }
+    }
+
+}