private static void reduce(byte[] s) {
    // Observation:
    // 2^252 mod l is equivalent to -27742317777372353535851937790883648493 mod l
    // Let m = -27742317777372353535851937790883648493
    // Thus a*2^252+b mod l is equivalent to a*m+b mod l
    //
    // First s is divided into chunks of 21 bits as follows:
    // s0+2^21*s1+2^42*s3+...+2^462*s23 = s[0]+256*s[1]+...+256^63*s[63]
    long s0 = 2097151 & load3(s, 0);
    long s1 = 2097151 & (load4(s, 2) >> 5);
    long s2 = 2097151 & (load3(s, 5) >> 2);
    long s3 = 2097151 & (load4(s, 7) >> 7);
    long s4 = 2097151 & (load4(s, 10) >> 4);
    long s5 = 2097151 & (load3(s, 13) >> 1);
    long s6 = 2097151 & (load4(s, 15) >> 6);
    long s7 = 2097151 & (load3(s, 18) >> 3);
    long s8 = 2097151 & load3(s, 21);
    long s9 = 2097151 & (load4(s, 23) >> 5);
    long s10 = 2097151 & (load3(s, 26) >> 2);
    long s11 = 2097151 & (load4(s, 28) >> 7);
    long s12 = 2097151 & (load4(s, 31) >> 4);
    long s13 = 2097151 & (load3(s, 34) >> 1);
    long s14 = 2097151 & (load4(s, 36) >> 6);
    long s15 = 2097151 & (load3(s, 39) >> 3);
    long s16 = 2097151 & load3(s, 42);
    long s17 = 2097151 & (load4(s, 44) >> 5);
    long s18 = 2097151 & (load3(s, 47) >> 2);
    long s19 = 2097151 & (load4(s, 49) >> 7);
    long s20 = 2097151 & (load4(s, 52) >> 4);
    long s21 = 2097151 & (load3(s, 55) >> 1);
    long s22 = 2097151 & (load4(s, 57) >> 6);
    long s23 = (load4(s, 60) >> 3);
    long carry0;
    long carry1;
    long carry2;
    long carry3;
    long carry4;
    long carry5;
    long carry6;
    long carry7;
    long carry8;
    long carry9;
    long carry10;
    long carry11;
    long carry12;
    long carry13;
    long carry14;
    long carry15;
    long carry16;

    // s23*2^462 = s23*2^210*2^252 is equivalent to s23*2^210*m in mod l
    // As m is a 125 bit number, the result needs to scattered to 6 limbs (125/21 ceil is 6)
    // starting from s11 (s11*2^210)
    // m = [666643, 470296, 654183, -997805, 136657, -683901] in 21-bit limbs
    s11 += s23 * 666643;
    s12 += s23 * 470296;
    s13 += s23 * 654183;
    s14 -= s23 * 997805;
    s15 += s23 * 136657;
    s16 -= s23 * 683901;
    // s23 = 0;

    s10 += s22 * 666643;
    s11 += s22 * 470296;
    s12 += s22 * 654183;
    s13 -= s22 * 997805;
    s14 += s22 * 136657;
    s15 -= s22 * 683901;
    // s22 = 0;

    s9 += s21 * 666643;
    s10 += s21 * 470296;
    s11 += s21 * 654183;
    s12 -= s21 * 997805;
    s13 += s21 * 136657;
    s14 -= s21 * 683901;
    // s21 = 0;

    s8 += s20 * 666643;
    s9 += s20 * 470296;
    s10 += s20 * 654183;
    s11 -= s20 * 997805;
    s12 += s20 * 136657;
    s13 -= s20 * 683901;
    // s20 = 0;

    s7 += s19 * 666643;
    s8 += s19 * 470296;
    s9 += s19 * 654183;
    s10 -= s19 * 997805;
    s11 += s19 * 136657;
    s12 -= s19 * 683901;
    // s19 = 0;

    s6 += s18 * 666643;
    s7 += s18 * 470296;
    s8 += s18 * 654183;
    s9 -= s18 * 997805;
    s10 += s18 * 136657;
    s11 -= s18 * 683901;
    // s18 = 0;

    // Reduce the bit length of limbs from s6 to s15 to 21-bits.
    carry6 = (s6 + (1 << 20)) >> 21; s7 += carry6; s6 -= carry6 << 21;
    carry8 = (s8 + (1 << 20)) >> 21; s9 += carry8; s8 -= carry8 << 21;
    carry10 = (s10 + (1 << 20)) >> 21; s11 += carry10; s10 -= carry10 << 21;
    carry12 = (s12 + (1 << 20)) >> 21; s13 += carry12; s12 -= carry12 << 21;
    carry14 = (s14 + (1 << 20)) >> 21; s15 += carry14; s14 -= carry14 << 21;
    carry16 = (s16 + (1 << 20)) >> 21; s17 += carry16; s16 -= carry16 << 21;

    carry7 = (s7 + (1 << 20)) >> 21; s8 += carry7; s7 -= carry7 << 21;
    carry9 = (s9 + (1 << 20)) >> 21; s10 += carry9; s9 -= carry9 << 21;
    carry11 = (s11 + (1 << 20)) >> 21; s12 += carry11; s11 -= carry11 << 21;
    carry13 = (s13 + (1 << 20)) >> 21; s14 += carry13; s13 -= carry13 << 21;
    carry15 = (s15 + (1 << 20)) >> 21; s16 += carry15; s15 -= carry15 << 21;

    // Resume reduction where we left off.
    s5 += s17 * 666643;
    s6 += s17 * 470296;
    s7 += s17 * 654183;
    s8 -= s17 * 997805;
    s9 += s17 * 136657;
    s10 -= s17 * 683901;
    // s17 = 0;

    s4 += s16 * 666643;
    s5 += s16 * 470296;
    s6 += s16 * 654183;
    s7 -= s16 * 997805;
    s8 += s16 * 136657;
    s9 -= s16 * 683901;
    // s16 = 0;

    s3 += s15 * 666643;
    s4 += s15 * 470296;
    s5 += s15 * 654183;
    s6 -= s15 * 997805;
    s7 += s15 * 136657;
    s8 -= s15 * 683901;
    // s15 = 0;

    s2 += s14 * 666643;
    s3 += s14 * 470296;
    s4 += s14 * 654183;
    s5 -= s14 * 997805;
    s6 += s14 * 136657;
    s7 -= s14 * 683901;
    // s14 = 0;

    s1 += s13 * 666643;
    s2 += s13 * 470296;
    s3 += s13 * 654183;
    s4 -= s13 * 997805;
    s5 += s13 * 136657;
    s6 -= s13 * 683901;
    // s13 = 0;

    s0 += s12 * 666643;
    s1 += s12 * 470296;
    s2 += s12 * 654183;
    s3 -= s12 * 997805;
    s4 += s12 * 136657;
    s5 -= s12 * 683901;
    s12 = 0;

    // Reduce the range of limbs from s0 to s11 to 21-bits.
    carry0 = (s0 + (1 << 20)) >> 21; s1 += carry0; s0 -= carry0 << 21;
    carry2 = (s2 + (1 << 20)) >> 21; s3 += carry2; s2 -= carry2 << 21;
    carry4 = (s4 + (1 << 20)) >> 21; s5 += carry4; s4 -= carry4 << 21;
    carry6 = (s6 + (1 << 20)) >> 21; s7 += carry6; s6 -= carry6 << 21;
    carry8 = (s8 + (1 << 20)) >> 21; s9 += carry8; s8 -= carry8 << 21;
    carry10 = (s10 + (1 << 20)) >> 21; s11 += carry10; s10 -= carry10 << 21;

    carry1 = (s1 + (1 << 20)) >> 21; s2 += carry1; s1 -= carry1 << 21;
    carry3 = (s3 + (1 << 20)) >> 21; s4 += carry3; s3 -= carry3 << 21;
    carry5 = (s5 + (1 << 20)) >> 21; s6 += carry5; s5 -= carry5 << 21;
    carry7 = (s7 + (1 << 20)) >> 21; s8 += carry7; s7 -= carry7 << 21;
    carry9 = (s9 + (1 << 20)) >> 21; s10 += carry9; s9 -= carry9 << 21;
    carry11 = (s11 + (1 << 20)) >> 21; s12 += carry11; s11 -= carry11 << 21;

    s0 += s12 * 666643;
    s1 += s12 * 470296;
    s2 += s12 * 654183;
    s3 -= s12 * 997805;
    s4 += s12 * 136657;
    s5 -= s12 * 683901;
    s12 = 0;

    // Carry chain reduction to propagate excess bits from s0 to s5 to the most significant limbs.
    carry0 = s0 >> 21; s1 += carry0; s0 -= carry0 << 21;
    carry1 = s1 >> 21; s2 += carry1; s1 -= carry1 << 21;
    carry2 = s2 >> 21; s3 += carry2; s2 -= carry2 << 21;
    carry3 = s3 >> 21; s4 += carry3; s3 -= carry3 << 21;
    carry4 = s4 >> 21; s5 += carry4; s4 -= carry4 << 21;
    carry5 = s5 >> 21; s6 += carry5; s5 -= carry5 << 21;
    carry6 = s6 >> 21; s7 += carry6; s6 -= carry6 << 21;
    carry7 = s7 >> 21; s8 += carry7; s7 -= carry7 << 21;
    carry8 = s8 >> 21; s9 += carry8; s8 -= carry8 << 21;
    carry9 = s9 >> 21; s10 += carry9; s9 -= carry9 << 21;
    carry10 = s10 >> 21; s11 += carry10; s10 -= carry10 << 21;
    carry11 = s11 >> 21; s12 += carry11; s11 -= carry11 << 21;

    // Do one last reduction as s12 might be 1.
    s0 += s12 * 666643;
    s1 += s12 * 470296;
    s2 += s12 * 654183;
    s3 -= s12 * 997805;
    s4 += s12 * 136657;
    s5 -= s12 * 683901;
    // s12 = 0;

    carry0 = s0 >> 21; s1 += carry0; s0 -= carry0 << 21;
    carry1 = s1 >> 21; s2 += carry1; s1 -= carry1 << 21;
    carry2 = s2 >> 21; s3 += carry2; s2 -= carry2 << 21;
    carry3 = s3 >> 21; s4 += carry3; s3 -= carry3 << 21;
    carry4 = s4 >> 21; s5 += carry4; s4 -= carry4 << 21;
    carry5 = s5 >> 21; s6 += carry5; s5 -= carry5 << 21;
    carry6 = s6 >> 21; s7 += carry6; s6 -= carry6 << 21;
    carry7 = s7 >> 21; s8 += carry7; s7 -= carry7 << 21;
    carry8 = s8 >> 21; s9 += carry8; s8 -= carry8 << 21;
    carry9 = s9 >> 21; s10 += carry9; s9 -= carry9 << 21;
    carry10 = s10 >> 21; s11 += carry10; s10 -= carry10 << 21;

    // Serialize the result into the s.
    s[0] = (byte) s0;
    s[1] = (byte) (s0 >> 8);
    s[2] = (byte) ((s0 >> 16) | (s1 << 5));
    s[3] = (byte) (s1 >> 3);
    s[4] = (byte) (s1 >> 11);
    s[5] = (byte) ((s1 >> 19) | (s2 << 2));
    s[6] = (byte) (s2 >> 6);
    s[7] = (byte) ((s2 >> 14) | (s3 << 7));
    s[8] = (byte) (s3 >> 1);
    s[9] = (byte) (s3 >> 9);
    s[10] = (byte) ((s3 >> 17) | (s4 << 4));
    s[11] = (byte) (s4 >> 4);
    s[12] = (byte) (s4 >> 12);
    s[13] = (byte) ((s4 >> 20) | (s5 << 1));
    s[14] = (byte) (s5 >> 7);
    s[15] = (byte) ((s5 >> 15) | (s6 << 6));
    s[16] = (byte) (s6 >> 2);
    s[17] = (byte) (s6 >> 10);
    s[18] = (byte) ((s6 >> 18) | (s7 << 3));
    s[19] = (byte) (s7 >> 5);
    s[20] = (byte) (s7 >> 13);
    s[21] = (byte) s8;
    s[22] = (byte) (s8 >> 8);
    s[23] = (byte) ((s8 >> 16) | (s9 << 5));
    s[24] = (byte) (s9 >> 3);
    s[25] = (byte) (s9 >> 11);
    s[26] = (byte) ((s9 >> 19) | (s10 << 2));
    s[27] = (byte) (s10 >> 6);
    s[28] = (byte) ((s10 >> 14) | (s11 << 7));
    s[29] = (byte) (s11 >> 1);
    s[30] = (byte) (s11 >> 9);
    s[31] = (byte) (s11 >> 17);
  }
Input:
s[0]+256*s[1]+...+256^63*s[63] = s

Output:
s[0]+256*s[1]+...+256^31*s[31] = s mod l
where l = 2^252 + 27742317777372353535851937790883648493.
Overwrites s in place.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java