adding-solutions.md

Adding a New Solution

1. Find or compute the answer

Add the answer to answers.txt:

NNN answer

Integer answers go on one line. Floating-point answers should match the precision Project Euler expects (usually 10-12 significant digits).

2. Write the Flow solution

Create problems/pNNN.flow. The file must define main() and print the answer as the last non-empty line.

# Project Euler NNN
# Brief description.

function solve() -> i64 {
    # ... your algorithm ...
    return 42
}

function main() -> i32 {
    printf("%lld\n", solve())
    return 0
}

Type conventions

• Integer answers: i64, print with %lld
• Floating-point answers: f64, print with %.10f or %.12e
• Large integers (beyond i64): print from native code directly

3. Test

./scripts/run.sh NNN

Output should be pNNN: <answer>.

4. Verify

./scripts/verify.sh NNN NNN

5. Native helper (if needed)

If the algorithm needs __int128, GMP, MPFR, NTT, or other facilities not available in Flow, create a native C helper.

Create problems/native/pNNN.c:

#include <stdint.h>

long long pNNN_native(void) {
    /* ... algorithm ... */
    return 42;
}

Update problems/pNNN.flow to call it:

extern {
    function pNNN_native() -> i64
}

function main() -> i32 {
    printf("%lld\n", pNNN_native())
    return 0
}

Floating-point native helpers

double pNNN_native(void) {
    return 3.14159265358979;
}

extern {
    function pNNN_native() -> f64
}

function main() -> i32 {
    printf("%.10f\n", pNNN_native())
    return 0
}

C++ helpers

Name the file problems/native/pNNN.cpp. The build system compiles the Flow-generated C as C, then links with the C++ TU. Use extern "C" for the exported function.

GMP / MPFR

The build system auto-detects Homebrew gmp and mpfr and links them if the headers are present at /opt/homebrew/include/. Just include the headers in your C file.

Answers exceeding i64

For answers that don't fit in i64 (e.g. p974's 29-digit integer), have the native code print the answer directly and return 0:

void pNNN_print(void) {
    /* compute and print the answer */
    printf("13313751171933973557517973175\n");
}

extern {
    function pNNN_print()
}

function main() -> i32 {
    pNNN_print()
    return 0
}

6. Data files

If the problem needs input data, put it in data/ with a descriptive name (e.g. data/p673_beds.txt). Reference it from the native helper using a relative path.

7. Update README

Update the progress table in README.md if the solution count changed.