Avoid using FromStr for parsing floats

f32s FromStr impl is very bloated, although probably faster than this
solution. This solution results in about 18KB less flash usage for the
nrf sample crate though.

Author: hulthe

Cargo.toml

@@ -1,29 +1,33 @@
-[package]
-name = "sim7000-async"
-version = "5.0.0"
-authors = [
-	"Zoey Riordan <[email protected]>",
-	"Joakim Hulthe <[email protected]>",
-]
-description = "Drivers for the SIM7000 series of chips"
-license = "Apache-2.0 OR MIT"
-repository = "https://github.com/technocreatives/sim7000"
-edition = "2021"
-rust-version = "1.75"
-
-[dependencies]
-critical-section = "1.1.2"
-defmt = { version = "0.3.2", optional = true }
-embassy-executor = "0.5.0"
-embassy-futures = "0.1.1"
-embassy-sync = "0.5.0"
-embassy-time = "0.3.0"
-embedded-io-async = "0.6.0"
-futures = { version = "0.3", default-features = false, features = ["async-await"] }
-heapless = "0.7"
-log = { version = "0.4", optional = true }
-
-[features]
-default = ["log"]
-log = ["dep:log"]
-defmt = ["dep:defmt",  "embassy-time/defmt", "heapless/defmt-impl"]
+[package]
+name = "sim7000-async"
+version = "5.0.0"
+authors = [
+	"Zoey Riordan <[email protected]>",
+	"Joakim Hulthe <[email protected]>",
+]
+description = "Drivers for the SIM7000 series of chips"
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/technocreatives/sim7000"
+edition = "2021"
+rust-version = "1.75"
+
+[dependencies]
+critical-section = "1.1.2"
+defmt = { version = "0.3.2", optional = true }
+embassy-executor = "0.5.0"
+embassy-futures = "0.1.1"
+embassy-sync = "0.5.0"
+embassy-time = "0.3.0"
+embedded-io-async = "0.6.0"
+futures = { version = "0.3", default-features = false, features = ["async-await"] }
+heapless = "0.7"
+log = { version = "0.4", optional = true }
+
+[features]
+default = ["log"]
+log = ["dep:log"]
+defmt = ["dep:defmt",  "embassy-time/defmt", "heapless/defmt-impl"]
+
+[dev-dependencies]
+quickcheck = "1.0.3"
+quickcheck_macros = "1.0.0"

src/at_command/mod.rs

@@ -1,6 +1,6 @@
 use core::{
     fmt::Debug,
-    num::{ParseFloatError, ParseIntError},
+    num::{IntErrorKind, ParseFloatError, ParseIntError},
 };
 
 pub mod generic_response;
@@ -180,6 +180,14 @@ impl From<&'static str> for AtParseErr {
     }
 }
 
+impl From<IntErrorKind> for AtParseErr {
+    fn from(_: IntErrorKind) -> Self {
+        AtParseErr {
+            message: "Failed to parse integer",
+        }
+    }
+}
+
 impl From<ParseIntError> for AtParseErr {
     fn from(_: ParseIntError) -> Self {
         AtParseErr {

src/at_command/unsolicited/ugnsinf.rs

@@ -1,6 +1,7 @@
 use core::str::FromStr;
 
 use crate::at_command::{AtParseErr, AtParseLine};
+use crate::parse_f32;
 use crate::util::collect_array;
 
 #[derive(Debug, PartialEq)]
@@ -55,18 +56,23 @@ impl AtParseLine for GnssReport {
                 .or_else(|e| s.is_empty().then(T::default).ok_or(e))
         }
 
+        // avoid parsing f32s with FromStr implementation because it uses 10KiB extra flash
+        fn parse_optional_f32(s: &str) -> Result<f32, AtParseErr> {
+            Ok(parse_f32(s).or_else(|e| s.is_empty().then_some(0.0).ok_or(e))?)
+        }
+
         Ok(GnssReport::Fix(GnssFix {
-            latitude: latitude.parse()?,
-            longitude: longitude.parse()?,
-            altitude: msl_altitude.parse()?,
+            latitude: parse_f32(latitude)?,
+            longitude: parse_f32(longitude)?,
+            altitude: parse_f32(msl_altitude)?,
 
             // The docs are unclear on what fields are optional, so just assume everything except
             // the core values are.
             speed_over_ground: parse_optional(speed_over_groud)?,
-            course_over_ground: parse_optional(course_over_ground)?,
-            hdop: parse_optional(hdop)?,
-            pdop: parse_optional(pdop)?,
-            vdop: parse_optional(vdop)?,
+            course_over_ground: parse_optional_f32(course_over_ground)?,
+            hdop: parse_optional_f32(hdop)?,
+            pdop: parse_optional_f32(pdop)?,
+            vdop: parse_optional_f32(vdop)?,
             signal_noise_ratio: parse_optional(c_n0_max)?,
 
             // The docs contradicts itself on what these values are and what they are called

src/util.rs

@@ -8,6 +8,8 @@ use core::{
 use embassy_sync::{blocking_mutex, blocking_mutex::raw::RawMutex, waitqueue::WakerRegistration};
 use heapless::Deque;
 
+use crate::at_command::AtParseErr;
+
 #[track_caller]
 pub(crate) fn collect_array<T: Default + Copy, const N: usize>(
     mut iter: impl Iterator<Item = T>,
@@ -19,6 +21,46 @@ pub(crate) fn collect_array<T: Default + Copy, const N: usize>(
     Some(out)
 }
 
+/// A naive float parsing implementation, made to be less bloated than the FromStr impl for f64s.
+///
+/// Only supports basic decimal numbers that look like `-?\d+(.\d*)?`.
+/// Does very litte sanity checking, may produce nonsensical results if given malformed strings.
+pub(crate) fn parse_f64(s: &str) -> Result<f64, &'static str> {
+    let (int, frac) = s
+        .find('.')
+        .map(|decimal_place| {
+            let (int, frac) = s.split_at(decimal_place);
+            let frac = &frac[1..];
+            let frac = frac.trim_end_matches('0');
+
+            (int, frac)
+        })
+        .unwrap_or((s, ""));
+
+    const PARSE_ERR: &str = "float parse error";
+    const OVERFLOW_ERR: &str = "float parse overflow";
+
+    let decimal_place = frac.len() as u32;
+
+    let whole: i64 = if int.is_empty() { Ok(0) } else { int.parse() }.map_err(|_| PARSE_ERR)?;
+    let frac: u64 = if frac.is_empty() { Ok(0) } else { frac.parse() }.map_err(|_| PARSE_ERR)?;
+
+    let mut frac = i64::try_from(frac).map_err(|_| OVERFLOW_ERR)?;
+    if whole.is_negative() {
+        frac = -frac;
+    }
+    let (whole, frac) = (whole as f64, frac as f64);
+
+    let pow = 10u32.pow(decimal_place) as f64;
+    let num = whole + frac / pow;
+    Ok(num)
+}
+
+/// Shorthand for [parse_f64] as `f32`.
+pub(crate) fn parse_f32(s: &str) -> Result<f32, AtParseErr> {
+    Ok(parse_f64(s)? as f32)
+}
+
 /// A signal with that keeps track of the last value signaled.
 pub struct StateSignal<M: RawMutex, T> {
     inner: blocking_mutex::Mutex<M, RefCell<StateSignalInner<T>>>,
@@ -154,3 +196,34 @@ impl<M: RawMutex, T: Debug, const N: usize> RingChannel<M, T, N> {
         });
     }
 }
+
+#[cfg(test)]
+mod test {
+    use core::fmt::Write;
+    use heapless::String;
+    use quickcheck_macros::quickcheck;
+
+    #[quickcheck]
+    // we use ints here because quickcheck with floats is broken
+    fn parse_f64(int: i16, frac: u16) {
+        let mut s = String::<128>::new();
+        write!(&mut s, "{int}.{frac}").expect("buffer overflow when stringifying float");
+        let s = s.trim();
+
+        let parsed = super::parse_f64(s).expect("failed to parse f64");
+        let parsed2 = s.parse().unwrap();
+        assert_eq!(parsed, parsed2);
+    }
+
+    #[quickcheck]
+    // we use ints here because quickcheck with floats is broken
+    fn parse_f32(int: i16, frac: u16) {
+        let mut s = String::<128>::new();
+        write!(&mut s, "{int}.{frac}").expect("buffer overflow when stringifying float");
+        let s = s.trim();
+
+        let parsed = super::parse_f32(s).expect("failed to parse f32");
+        let parsed2 = s.parse().unwrap();
+        assert_eq!(parsed, parsed2);
+    }
+}