From b0c68308b7da7bb17924929076a4f25e450218ff Mon Sep 17 00:00:00 2001
From: Colin <colin@uninsane.org>
Date: Sat, 29 Apr 2023 03:22:10 +0000
Subject: [PATCH] WIP: mx-sanebot: first pass at formalizing a parser

---
 pkgs/mx-sanebot/src/main.rs        |  1 +
 pkgs/mx-sanebot/src/msg_handler.rs | 44 +++++++++++++++++
 pkgs/mx-sanebot/src/parsing.rs     | 79 ++++++++++++++++++++++++++++++
 3 files changed, 124 insertions(+)
 create mode 100644 pkgs/mx-sanebot/src/parsing.rs

diff --git a/pkgs/mx-sanebot/src/main.rs b/pkgs/mx-sanebot/src/main.rs
index 6e0e4edc..e7ae9d2d 100644
--- a/pkgs/mx-sanebot/src/main.rs
+++ b/pkgs/mx-sanebot/src/main.rs
@@ -1,4 +1,5 @@
 mod msg_handler;
+mod parsing;
 
 use std::env;
 
diff --git a/pkgs/mx-sanebot/src/msg_handler.rs b/pkgs/mx-sanebot/src/msg_handler.rs
index 013c4503..f4f523dc 100644
--- a/pkgs/mx-sanebot/src/msg_handler.rs
+++ b/pkgs/mx-sanebot/src/msg_handler.rs
@@ -3,6 +3,41 @@ use std::fmt;
 use std::process;
 use std::str;
 
+use super::parsing;
+
+
+mod tt {
+    pub(super) use super::parsing::{
+        Either,
+        Lit,
+        Then,
+    };
+
+    // grammar:
+    // REQUEST = <!> (HELP | BT | BT-ADD)
+    // HELP = <help>
+    // BT = <bt>
+    // BT-ADD = <bt-add> MAYBE_ARGS
+    // MAYBE_ARGS = [SPACE [ARGS]]
+    // ARGS = ARG MAYBE_ARGS
+    // ARG = (not SPACE) [ARG]
+    pub(super) type Request = Then<Bang, Either<Help, Bt>>;
+
+    pub(super) type Bang = Lit<{ '!' as u8 }>;
+
+    pub(super) type Help = Then<
+        Lit<{ 'H' as u8 }>, Then<
+        Lit<{ 'E' as u8 }>, Then<
+        Lit<{ 'L' as u8 }>,
+        Lit<{ 'P' as u8 }>,
+    >>>;
+
+    pub(super) type Bt = Then<
+        Lit<{ 'B' as u8 }>,
+        Lit<{ 'T' as u8 }>,
+    >;
+}
+
 pub struct MessageHandler;
 
 impl MessageHandler {
@@ -31,6 +66,15 @@ enum Request {
     Bt,
 }
 
+impl From<tt::Request> for Request {
+    fn from(t: tt::Request) -> Self {
+        match t {
+            tt::Then(_bang, tt::Either::A(_help)) => Self::Help,
+            tt::Then(_bang, tt::Either::B(_bt)) => Self::Bt,
+        }
+    }
+}
+
 impl Request {
     fn evaluate(self) -> Response {
         match self {
diff --git a/pkgs/mx-sanebot/src/parsing.rs b/pkgs/mx-sanebot/src/parsing.rs
new file mode 100644
index 00000000..4e75f039
--- /dev/null
+++ b/pkgs/mx-sanebot/src/parsing.rs
@@ -0,0 +1,79 @@
+/// for internal use.
+/// parses only if the parser has no more bytes to yield.
+struct Eof;
+
+/// literal byte (character).
+pub struct Lit<const BYTE: u8>;
+
+/// the two-item sequence of A followed by B.
+pub struct Then<A, B>(pub A, pub B);
+
+/// if A parses, then A, else parse B.
+pub enum Either<A, B> {
+    A(A),
+    B(B),
+}
+
+// case-insensitive u8 character.
+// type ILit<const BYTE: u8> = Either<Lit<{ BYTE.to_ascii_lowercase() }>, Lit<{ BYTE.to_ascii_uppercase() }>>;
+
+
+pub type PResult<P, C> = std::result::Result<(C, P), P>;
+pub trait Parser: Sized {
+    fn expect_byte(self, b: Option<u8>) -> PResult<Self, ()>;
+    fn expect<C: Parse>(self) -> PResult<Self, C>;
+    // {
+    //     // support backtracking; i.e. don't modify `self` on failed parse
+    //     match C::consume(self.clone()) {
+    //          Ok(res) => res,
+    //          Err(_) => self,
+    //     }
+    // }
+    fn parse_all<C: Parse>(self) -> Result<C, ()> {
+        match self.expect::<Then<C, Eof>>() {
+            Ok((Then(c, _eof), _p)) => Ok(c),
+            Err(_p) => Err(()),
+        }
+    }
+}
+
+pub trait Parse: Sized {
+    fn consume<P: Parser>(p: P) -> PResult<P, Self>;
+}
+
+impl Parse for Eof {
+    fn consume<P: Parser>(p: P) -> PResult<P, Self> {
+        let (_, p) = p.expect_byte(None)?;
+        Ok((Self, p))
+    }
+}
+
+impl<const BYTE: u8> Parse for Lit<BYTE> {
+    fn consume<P: Parser>(p: P) -> PResult<P, Self> {
+        let (_, p) = p.expect_byte(Some(BYTE))?;
+        Ok((Self, p))
+    }
+}
+
+impl<A: Parse, B: Parse> Parse for Then<A, B> {
+    fn consume<P: Parser>(p: P) -> PResult<P, Self> {
+        let (a, p) = p.expect()?;
+        let (b, p) = p.expect()?;
+        Ok((Self(a, b), p))
+    }
+}
+
+impl<A: Parse, B: Parse> Parse for Either<A, B> {
+    fn consume<P: Parser>(p: P) -> PResult<P, Self> {
+        let p = match p.expect() {
+            Ok((a, p)) => { return Ok((Self::A(a), p)); },
+            Err(p) => p,
+        };
+        let p = match p.expect() {
+            Ok((b, p)) => { return Ok((Self::B(b), p)); },
+            Err(p) => p,
+        };
+        Err(p)
+    }
+}
+