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Reduce allocations during encoding #11

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Reduce allocations during encoding #11

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8978c9c
Abstract over quantization function in encode_ac
Spaceface16518 Jan 22, 2021
1df6990
Abstract over rounding function in encode_dc
Spaceface16518 Jan 22, 2021
230cd86
Remove redundant parenthesis
Spaceface16518 Jan 22, 2021
5c0b127
Only allocate once when constructing hash
Spaceface16518 Jan 22, 2021
00558a3
Use associated `pow` function
Spaceface16518 Jan 22, 2021
5e2586c
Format code
Spaceface16518 Jan 22, 2021
90d464a
Don't clone ac vector when calculating max value
Spaceface16518 Jan 22, 2021
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70 changes: 26 additions & 44 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -203,9 +203,9 @@ pub fn encode(
bytes_per_pixel,
0,
|a, b| {
(normalisation
normalisation
* f64::cos((PI * x as f64 * a) / width as f64)
* f64::cos((PI * y as f64 * b) / height as f64))
* f64::cos((PI * y as f64 * b) / height as f64)
Comment on lines -206 to +208
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This just gets rid of a rustc warning

},
);

Expand All @@ -219,36 +219,36 @@ pub fn encode(
}
}

let mut hash = String::new();
let mut hash = String::with_capacity(1 + 1 + 4 + 2 * ac.len());

let size_flag = ((cx - 1) + (cy - 1) * 9) as usize;
hash += &encode_base83_string(size_flag, 1);
hash.extend(encode_base83_string(size_flag, 1));

let maximum_value: f64;

if !ac.is_empty() {
// I'm sure there's a better way to write this; following the Swift atm :)
let maxf = |a: &f64, b: &f64| a.partial_cmp(b).unwrap_or(Ordering::Equal);
let actual_maximum_value = ac
.clone()
.into_iter()
.map(|[a, b, c]| f64::max(f64::max(f64::abs(a), f64::abs(b)), f64::abs(c)))
.max_by(|a, b| a.partial_cmp(b).unwrap_or(Ordering::Equal))
.iter()
.map(|channels| channels.iter().copied().map(f64::abs).max_by(maxf).unwrap())
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Since we are iterating over an array, this will not be any less efficient than the hand-rolled version because it will likely be unrolled or vectorized.

.max_by(maxf)
.unwrap();
let quantised_maximum_value = usize::max(
0,
usize::min(82, f64::floor(actual_maximum_value * 166f64 - 0.5) as usize),
);
maximum_value = ((quantised_maximum_value + 1) as f64) / 166f64;
hash += &encode_base83_string(quantised_maximum_value, 1);
hash.extend(encode_base83_string(quantised_maximum_value, 1));
} else {
maximum_value = 1f64;
hash += &encode_base83_string(0, 1);
hash.extend(encode_base83_string(0, 1));
}

hash += &encode_base83_string(encode_dc(dc), 4);
hash.extend(encode_base83_string(encode_dc(dc), 4));

for factor in ac {
hash += &encode_base83_string(encode_ac(factor, maximum_value), 2);
hash.extend(encode_base83_string(encode_ac(factor, maximum_value), 2));
}

Ok(hash)
Expand Down Expand Up @@ -300,37 +300,20 @@ where
[r * scale, g * scale, b * scale]
}

fn encode_dc(value: [f64; 3]) -> usize {
let rounded_r = linear_to_srgb(value[0]);
let rounded_g = linear_to_srgb(value[1]);
let rounded_b = linear_to_srgb(value[2]);
((rounded_r << 16) + (rounded_g << 8) + rounded_b) as usize
fn encode_dc([r, g, b]: [f64; 3]) -> usize {
let rounded = |v| linear_to_srgb(v);
((rounded(r) << 16) + (rounded(g) << 8) + rounded(b)) as usize
}

fn encode_ac(value: [f64; 3], maximum_value: f64) -> usize {
let quant_r = f64::floor(f64::max(
0f64,
f64::min(
18f64,
f64::floor(sign_pow(value[0] / maximum_value, 0.5) * 9f64 + 9.5),
),
));
let quant_g = f64::floor(f64::max(
0f64,
f64::min(
18f64,
f64::floor(sign_pow(value[1] / maximum_value, 0.5) * 9f64 + 9.5),
),
));
let quant_b = f64::floor(f64::max(
0f64,
f64::min(
18f64,
f64::floor(sign_pow(value[2] / maximum_value, 0.5) * 9f64 + 9.5),
),
));

(quant_r * 19f64 * 19f64 + quant_g * 19f64 + quant_b) as usize
fn encode_ac([r, g, b]: [f64; 3], maximum_value: f64) -> usize {
let quant = |v| {
(sign_pow(v / maximum_value, 0.5) * 9. + 9.5)
.floor()
.min(18.)
.max(0.)
};

(quant(r) * 19f64 * 19f64 + quant(g) * 19f64 + quant(b)) as usize
Comment on lines -303 to +316
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Using a closure here drastically reduces the code size. It has the exact same behavior.

Using a closure will not have any overhead, since the virtual function does not escape the parent function.

}

// Base83
Expand All @@ -352,11 +335,10 @@ fn decode_base83_string(string: &str) -> usize {
.fold(0, |value, digit| value * 83 + digit)
}

fn encode_base83_string(value: usize, length: u32) -> String {
fn encode_base83_string(value: usize, length: u32) -> impl Iterator<Item = char> {
(1..=length)
.map(|i| (value / usize::pow(83, length - i)) % 83)
.map(move |i| (value / 83usize.pow(length - i)) % 83)
.map(|digit| ENCODE_CHARACTERS[digit])
.collect()
}

#[cfg(test)]
Expand Down