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Reduce the number of string allocations done during serialisation #41

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563f8c0
Reduce String allocations
TomFryers Nov 11, 2025
876a8f7
Use fmt::Write for serialising LispObjects
TomFryers Nov 11, 2025
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94 changes: 50 additions & 44 deletions src/bytecode.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
use std::fmt::Write;
use std::collections::BTreeMap;
use std::str::FromStr;

Expand Down Expand Up @@ -35,77 +36,82 @@ impl FromStr for LispObject {
}
}

impl LispObject {
fn to_repl(&self) -> String {
impl std::fmt::Display for LispObject {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
LispObject::Symbol(s) => s.clone(),
LispObject::Keyword(s) => format!(":{}", s),
LispObject::Symbol(s) => f.write_str(s),
LispObject::Keyword(s) => write!(f, ":{s}"),
LispObject::Str(s) => {
let mut result = String::new();
result.reserve(s.len() * 2 + 2);
result.push('"');
f.write_char('"')?;
for c in s.chars() {
if c == '\"' || c == '\\' {
result.push('\\');
result.push(c);
f.write_char('\\')?;
f.write_char(c)?;
} else if (c as u32) < 32 || (c as u32) == 127 {
// not printable
// NOTE: cannot use escape for c in 128..=255, otherwise the string would become unibyte
result += &format!("\\{:03o}", c as u32);
write!(f, "\\{:03o}", c as u32)?;
} else {
result.push(c);
f.write_char(c)?;
}
}
result.push('"');
result
f.write_char('"')?;
Ok(())
},
LispObject::UnibyteStr(vec) => {
let mut result = String::new();
result.reserve(vec.len() * 4 + 2);
result.push('"');
f.write_char('"')?;
let mut last_oct_escape_not_full = false;
for c in vec {
let mut oct_escape_not_full = false;
match *c {
7 => result += "\\a",
8 => result += "\\b",
9 => result += "\\t",
10 => result += "\\n",
11 => result += "\\v",
12 => result += "\\f",
13 => result += "\\r",
127 => result += "\\d",
27 => result += "\\e",
7 => f.write_str("\\a")?,
8 => f.write_str("\\b")?,
9 => f.write_str("\\t")?,
10 => f.write_str("\\n")?,
11 => f.write_str("\\v")?,
12 => f.write_str("\\f")?,
13 => f.write_str("\\r")?,
127 => f.write_str("\\d")?,
27 => f.write_str("\\e")?,
// NOTE: do not use 0..=7 in this branch, because it takes one more byte than the next branch
8..=26 => { // \^@ \^A \^B ... \^Z
result += &format!("\\^{}", (*c as u32 + 64) as u8 as char);
write!(f, "\\^{}", (*c as u32 + 64) as u8 as char)?;
},
0..=7 | 27..=31 | 128..=255 | 34 | 92 => { // oct, for unprintable and '"' and '\\'
let oct_s = format!("\\{:o}", *c as u32);
if oct_s.len() < 4 {
oct_escape_not_full = true;
}
result += &oct_s;
f.write_str(&oct_s)?;
},
_ => { // printable
// https://www.gnu.org/software/emacs/manual/html_node/elisp/Non_002dASCII-in-St
if last_oct_escape_not_full && ('0'..='7').contains(&(*c as char)) {
result += "\\ ";
f.write_str("\\ ")?;
}
result.push(*c as char);
f.write_char(*c as char)?;
},
}
last_oct_escape_not_full = oct_escape_not_full;
}
result.push('"');
result
f.write_char('"')
},
LispObject::Int(i) => i.to_string(),
LispObject::Float(s) => s.clone(),
LispObject::Nil => "nil".into(),
LispObject::T => "t".into(),
LispObject::Vector(v) =>
format!("[{}]", v.iter().map(|x| x.to_repl()).collect::<Vec<_>>().join(" "))
LispObject::Int(i) => write!(f, "{i}"),
LispObject::Float(s) => write!(f, "{s}"),
LispObject::Nil => f.write_str("nil"),
LispObject::T => f.write_str("t"),
LispObject::Vector(v) => {
f.write_char('[')?;
let mut iter = v.iter();
if let Some(first) = iter.next() {
write!(f, "{first}")?;
for x in iter {
write!(f, " {x}")?;
}
}
f.write_char(']')?;
Ok(())
}
}
}
}
Expand Down Expand Up @@ -423,8 +429,8 @@ impl BytecodeCompiler {
fn into_repl(self) -> Result<String> {
let (code, constants, max_stack_size) = self.into_bytecode()?;
Ok(format!("#[0 {} {} {}]",
LispObject::UnibyteStr(code).to_repl(),
LispObject::Vector(constants).to_repl(),
LispObject::UnibyteStr(code),
LispObject::Vector(constants),
max_stack_size))
}
}
Expand All @@ -442,9 +448,9 @@ pub fn generate_bytecode_repl(value: &json::Value, options: BytecodeOptions) ->

#[test]
fn test_string_repl() {
assert_eq!(LispObject::UnibyteStr("\x00".into()).to_repl(), r#""\0""#);
assert_eq!(LispObject::UnibyteStr("\x1a".into()).to_repl(), r#""\^Z""#);
assert_eq!(LispObject::UnibyteStr("\x20".into()).to_repl(), r#"" ""#);
assert_eq!(LispObject::UnibyteStr("\x7f".into()).to_repl(), r#""\d""#);
assert_eq!(LispObject::UnibyteStr(vec![0xff]).to_repl(), r#""\377""#);
assert_eq!(format!("{}", LispObject::UnibyteStr("\x00".into())), r#""\0""#);
assert_eq!(format!("{}", LispObject::UnibyteStr("\x1a".into())), r#""\^Z""#);
assert_eq!(format!("{}", LispObject::UnibyteStr("\x20".into())), r#"" ""#);
assert_eq!(format!("{}", LispObject::UnibyteStr("\x7f".into())), r#""\d""#);
assert_eq!(format!("{}", LispObject::UnibyteStr(vec![0xff])), r#""\377""#);
}