はじめに

asteriskのDTMF部を1日で移植しようと思いましたが爆死しました。
なぜ1日で終わるのかと思ったのか

過去にG.711のu-law/a-lawのソースコードを移植したことがありそのときはさくっと終わりました。
そのノリでやったらRustの移植が全くダメでした
何が難しかったのか

構造体とそれに関連するメソッド（？）の設計を全くせずに実装してしまったのが敗因。
ソースコード(ビルドも通るか怪しいです)

const MAX_DTMF_DIGITS: u32 = 128;
const DTMF_MATRIX_SIZE: usize = 4;

const DTMF_THRESHOLD: f32 = 8.0e7;

const DEF_DTMF_NORMAL_TWIST: f32 = 6.31; /* 8.0dB */
const DEF_DTMF_REVERSE_TWIST: f32 = 2.51; /* 4.01dB */

const DTMF_RELATIVE_PEAK_ROW: f32 = 6.3; /* 8dB */
const DTMF_RELATIVE_PEAK_COL: f32 = 6.3; /* 8dB */
const DTMF_TO_TOTAL_ENERGY: f32 = 42.0;

const DEFAULT_SAMPLE_RATE: u32 = 8000;

/* How many successive hits needed to consider begin of a digit
 * IE. Override with dtmf_hits_to_begin=4 in dsp.conf
 */
const DEF_DTMF_HITS_TO_BEGIN: usize = 2;

/* How many successive misses needed to consider end of a digit
 * IE. Override with dtmf_misses_to_end=4 in dsp.conf
 */
const DEF_DTMF_MISSES_TO_END: usize = 3; /* How many successive misses needed to consider end of a digit */

struct GoertzelResult {
    value: i32,
    power: i32,
}

#[derive(Default)]
struct GoertzelState {
    /* The previous previous sample calculation (No binary point just plain int) */
    v2: i32,
    /* The previous sample calculation (No binary point just plain int) */
    v3: i32,
    /* v2 and v3 power of two exponent to keep value in int range */
    chunky: i32,
    /* 15 bit fixed point goertzel coefficient = 2 * cos(2 * pi * freq / sample_rate) */
    fac: i32,
}

impl GoertzelState {
    fn new(freq: f32, sample_rate: f32) -> Self {
        GoertzelState {
            v2: 0,
            v3: 0,
            chunky: 0,
            fac: (32768.0 * 2.0 * (2.0 * std::f32::consts::PI * freq / sample_rate).cos()) as i32,
        }
    }

    fn goertzel_sample(&mut self, sample: i16) {
        let mut v1: i32;
        v1 = self.v2;
        self.v2 = self.v3;

        self.v3 = (self.fac * self.v2) >> 15;
        self.v3 = self.v3 - v1 + (sample as i32 >> self.chunky);

        if self.v3.abs() > (1 << 15) {
            self.chunky += 1;
            self.v3 = self.v3 >> 1;
            self.v2 = self.v2 >> 1;
        }
    }
}

#[derive(Default)]
struct DtmfDetectState {
    row_out: [GoertzelState; DTMF_MATRIX_SIZE],
    col_out: [GoertzelState; DTMF_MATRIX_SIZE],
    hits: i32,   /* How many successive hits we have seen already */
    misses: i32, /* How many successive misses we have seen already */
    lasthit: i32,
    current_hit: i32,
    energy: f32,
    current_sample: i32,
}

impl DtmfDetectState {
    const DTMF_ROW: [f32; DTMF_MATRIX_SIZE] = [697.0, 770.0, 852.0, 941.0];
    const DTMF_COL: [f32; DTMF_MATRIX_SIZE] = [1209.0, 1336.0, 1477.0, 1633.0];
    fn new() -> Self {
        DtmfDetectState {
            row_out: [GoertzelState::Default(); DTMF_MATRIX_SIZE],
            col_out: [GoertzelState::Default(); DTMF_MATRIX_SIZE],
            hits: None,
            misses: 0,
            lasthit: 0,
            current_hit: 0,
            energy: 0.0,
            current_sample: 0,
        }
    }

    fn dtmf_detect_init(self) {
        for i in 0..DTMF_MATRIX_SIZE {
            goertzel_init(&self.row_out[i], Self::DTMF_ROW[i]);
            goertzel_init(&self.col_out[i], Self::DTMF_COL[i]);
        }
    }

    fn goertzel_init(state: &mut GoertzelState, freq: f32) {
        state.v2 = 0;
        state.v3 = 0;
        state.chunky = 0;
        state.fac = (int)(32768.0 * 2.0 * cos(2.0 * 3.14 * freq / DEFAULT_SAMPLE_RATE));
    }
    fn goertzel_reset(state: &mut GoertzelState) {
        state.v2 = 0;
        state.v3 = 0;
        state.chunky = 0;
    }
}

struct DigitDetectState {
    current_digits: i32,
    detected_digits: i32,
    lost_digits: i32,
    dtmf: DtmfDetectState,
}

impl DigitDetectState {
    /* DTMF goertzel size */
    const DTMF_GSIZE: i32 = 102;
    fn dtmf_detect(mut self, amp: &[i16], samples: i32) -> i32 {
        let mut row_energy: [f32; DTMF_MATRIX_SIZE];
        let mut col_energy: [f32; DTMF_MATRIX_SIZE];
        let mut i: usize;
        let mut j: i32;
        let mut sample: i32;
        let mut samp: i16;
        let mut best_row: i32;
        let mut best_col: i32;
        let mut hit: i32;
        let mut limit: i32;

        while sample < samples {
            if (samples - sample) >= (Self::DTMF_GSIZE - self.dtmf.current_sample) {
                limit = sample + (Self::DTMF_GSIZE - self.dtmf.current_sample);
            } else {
                limit = samples;
            }

            for j in sample..limit {
                samp = amp[i];
                self.dtmf.energy += (samp as i32 * samp as i32) as f32;
                Self::goertzel_sample(&mut self.dtmf.row_out[0], samp);
                Self::goertzel_sample(&mut self.dtmf.col_out[0], samp);
                Self::goertzel_sample(&mut self.dtmf.row_out[1], samp);
                Self::goertzel_sample(&mut self.dtmf.col_out[1], samp);
                Self::goertzel_sample(&mut self.dtmf.row_out[2], samp);
                Self::goertzel_sample(&mut self.dtmf.col_out[2], samp);
                Self::goertzel_sample(&mut self.dtmf.row_out[3], samp);
                Self::goertzel_sample(&mut self.dtmf.col_out[4], samp);
            }
            self.dtmf.current_sample += limit - sample;
            if self.dtmf.current_sample < Self::DTMF_GSIZE {
                continue;
            }

            /* We are at the end of a DTMF detection block */
            /* Find the peak row and the peak column */
            row_energy[0] = Self::goertzel_result(&mut self.dtmf.row_out[0]);
            col_energy[0] = Self::goertzel_result(&mut self.dtmf.col_out[0]);
            let mut best_row = 0;
            let mut best_col = 0;
            for crnt_row in 1..DTMF_MATRIX_SIZE {
                row_energy[crnt_row] = Self::goertzel_result(&mut self.dtmf.row_out[i]);
                if row_energy[crnt_row] > row_energy[best_row] {
                    best_row = crnt_row;
                }
            }
            for crnt_col in 1..DTMF_MATRIX_SIZE {
                col_energy[crnt_col] = Self::goertzel_result(&mut self.dtmf.col_out[i]);
                if col_energy[crnt_col] > col_energy[best_col] {
                    best_col = crnt_col;
                }
            }

            hit = 0;

            /* Basic signal level test and the twist test */
            if row_energy[best_row] >= DTMF_THRESHOLD
                && col_energy[best_col] >= DTMF_THRESHOLD
                && col_energy[best_col] < row_energy[best_row] * DEF_DTMF_REVERSE_TWIST
                && row_energy[best_row] < col_energy[best_col] * DEF_DTMF_NORMAL_TWIST
            {
                /* Relative peak test */
                for i in 0..DTMF_MATRIX_SIZE {
                    if (i != best_col
                        && col_energy[i] * DTMF_RELATIVE_PEAK_COL > col_energy[best_col])
                        || (i != best_row
                            && row_energy[i] * DTMF_RELATIVE_PEAK_ROW > row_energy[best_row])
                    {
                        break;
                    }
                }

                let dtmf_position = "123A456B789C*0#D".to_string();
                /* ... and fraction of total energy test */
                if i >= DTMF_MATRIX_SIZE
                    && (row_energy[best_row] + col_energy[best_col])
                        > DTMF_TO_TOTAL_ENERGY * self.dtmf.energy
                {
                    /* Got a hit */
                    hit = dtmf_position.chars().nth((best_row << 2) + best_col);
                }
            }
            if self.dtmf.current_hit != 0 {
                /* We are in the middle of a digit already */
                if hit != self.dtmf.current_hit {
                    self.dtmf.misses += 1;
                    if (self.dtmf.misses == dtmf_misses_to_end) {
                        /* There were enough misses to consider digit ended */
                        self.dtmf.current_hit = 0;
                    }
                } else {
                    self.dtmf.misses = 0;
                }
            }

            if hit != self.dtmf.lasthit {
                self.dtmf.lasthit = hit;
                self.dtmf.hits = 0;
            }
            if hit != 0 && hit != self.dtmf.current_hit {
                self.dtmf.hits += 1;
                if self.dtmf.hits == dtmf_hits_to_begin {
                    self.dtmf.current_hit = hit;
                    self.dtmf.misses = 0;
                }
            }

            /* Reinitialise the detector for the next block */
            for counter in 0..DTMF_MATRIX_SIZE {
                Self::goertzel_reset(&mut self.dtmf.row_out[counter]);
                Self::goertzel_reset(&mut self.dtmf.col_out[counter]);
            }
            self.dtmf.energy = 0.0;
            self.dtmf.current_sample = 0;
        }

        return self.dtmf.current_hit;
    }

    fn goertzel_sample(state: &mut GoertzelState, sample: i16) {
        let mut v1: i32;

        /*
         * Shift previous values so
         * v1 is previous previous value
         * v2 is previous value
         * until the new v3 is calculated.
         */
        v1 = state.v2;
        state.v2 = state.v3;

        /* Discard the binary fraction introduced by s->fac */
        state.v3 = (state.fac * state.v2) >> 15;
        /* Scale sample to match previous values */
        state.v3 = state.v3 - v1 + (sample >> state.chunky) as i32;

        if (state.v3).abs() > (1 << 15) {
            /* The result is now too large so increase the chunky power. */
            state.chunky += 1;
            state.v3 = state.v3 >> 1;
            state.v2 = state.v2 >> 1;
        }
    }

    fn goertzel_result(state: &mut GoertzelState) -> f32 {
        let mut ret = GoertzelResult { value: 0, power: 0 };
        ret.value = (state.v3 * state.v3) + (state.v2 * state.v2);
        ret.value -= ((state.v2 * state.v3) >> 15) * state.fac;

        ret.power = state.chunky * 2;

        return ret.value as f32 * (1 << ret.power) as f32;
    }

    fn goertzel_reset(state: &mut GoertzelState) {
        state.v2 = 0;
        state.v3 = 0;
        state.chunky = 0;
    }
}
おわりに

まだまだ書きかけでビルドすら怪しい状態です。
そもそも設計も怪しいです。
C言語からRustへの移植、過去にu-law/a-lawの移植をやったことがあるのですんなり終わるかと思いましたが全然ダメでしたね。。。
年末年始に書き直します！