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diff --git a/src/Arithmetic/BarrettReduction/Wikipedia.v b/src/Arithmetic/BarrettReduction/Wikipedia.v new file mode 100644 index 000000000..69ce10c4b --- /dev/null +++ b/src/Arithmetic/BarrettReduction/Wikipedia.v @@ -0,0 +1,122 @@ +(*** Barrett Reduction *) +(** This file implements Barrett Reduction on [Z]. We follow Wikipedia. *) +Require Import Coq.ZArith.ZArith Coq.micromega.Psatz. +Require Import Crypto.Util.ZUtil. +Require Import Crypto.Util.Tactics.BreakMatch. + +Local Open Scope Z_scope. + +Section barrett. + Context (n a : Z) + (n_reasonable : n <> 0). + (** Quoting Wikipedia <https://en.wikipedia.org/wiki/Barrett_reduction>: *) + (** In modular arithmetic, Barrett reduction is a reduction + algorithm introduced in 1986 by P.D. Barrett. A naive way of + computing *) + (** [c = a mod n] *) + (** would be to use a fast division algorithm. Barrett reduction is + an algorithm designed to optimize this operation assuming [n] is + constant, and [a < n²], replacing divisions by + multiplications. *) + + (** * General idea *) + Section general_idea. + (** Let [m = 1 / n] be the inverse of [n] as a floating point + number. Then *) + (** [a mod n = a - ⌊a m⌋ n] *) + (** where [⌊ x ⌋] denotes the floor function. The result is exact, + as long as [m] is computed with sufficient accuracy. *) + + (* [/] is [Z.div], which means truncated division *) + Local Notation "⌊am⌋" := (a / n) (only parsing). + + Theorem naive_barrett_reduction_correct + : a mod n = a - ⌊am⌋ * n. + Proof using n_reasonable. + apply Zmod_eq_full; assumption. + Qed. + End general_idea. + + (** * Barrett algorithm *) + Section barrett_algorithm. + (** Barrett algorithm is a fixed-point analog which expresses + everything in terms of integers. Let [k] be the smallest + integer such that [2ᵏ > n]. Think of [n] as representing the + fixed-point number [n 2⁻ᵏ]. We precompute [m] such that [m = + ⌊4ᵏ / n⌋]. Then [m] represents the fixed-point number + [m 2⁻ᵏ ≈ (n 2⁻ᵏ)⁻¹]. *) + (** N.B. We don't need [k] to be the smallest such integer. *) + Context (k : Z) + (k_good : n < 2 ^ k) + (m : Z) + (m_good : m = 4^k / n). (* [/] is [Z.div], which is truncated *) + (** Wikipedia neglects to mention non-negativity, but we need it. + It might be possible to do with a relaxed assumption, such as + the sign of [a] and the sign of [n] being the same; but I + figured it wasn't worth it. *) + Context (n_pos : 0 < n) (* or just [0 <= n], since we have [n <> 0] above *) + (a_nonneg : 0 <= a). + + Lemma k_nonnegative : 0 <= k. + Proof using Type*. + destruct (Z_lt_le_dec k 0); try assumption. + rewrite !Z.pow_neg_r in * by lia; lia. + Qed. + + (** Now *) + Let q := (m * a) / 4^k. + Let r := a - q * n. + (** Because of the floor function (in Coq, because [/] means + truncated division), [q] is an integer and [r ≡ a mod n]. *) + Theorem barrett_reduction_equivalent + : r mod n = a mod n. + Proof using m_good. + subst r q m. + rewrite <- !Z.add_opp_r, !Zopp_mult_distr_l, !Z_mod_plus_full by assumption. + reflexivity. + Qed. + + Lemma qn_small + : q * n <= a. + Proof using a_nonneg k_good m_good n_pos n_reasonable. + pose proof k_nonnegative; subst q r m. + assert (0 <= 2^(k-1)) by zero_bounds. + Z.simplify_fractions_le. + Qed. + + (** Also, if [a < n²] then [r < 2n]. *) + (** N.B. It turns out that it is sufficient to assume [a < 4ᵏ]. *) + Context (a_small : a < 4^k). + Lemma q_nice : { b : bool | q = a / n + if b then -1 else 0 }. + Proof using a_nonneg a_small k_good m_good n_pos n_reasonable. + assert (0 <= (4 ^ k * a / n) mod 4 ^ k < 4 ^ k) by auto with zarith lia. + assert (0 <= a * (4 ^ k mod n) / n < 4 ^ k) by (auto with zero_bounds zarith lia). + subst q r m. + rewrite (Z.div_mul_diff_exact''' (4^k) n a) by lia. + rewrite (Z_div_mod_eq (4^k * _ / n) (4^k)) by lia. + autorewrite with push_Zmul push_Zopp zsimplify zstrip_div. + eexists; reflexivity. + Qed. + + Lemma r_small : r < 2 * n. + Proof using a_nonneg a_small k_good m_good n_pos n_reasonable q. + Hint Rewrite (Z.mul_div_eq' a n) using lia : zstrip_div. + assert (a mod n < n) by auto with zarith lia. + unfold r; rewrite (proj2_sig q_nice); generalize (proj1_sig q_nice); intro; subst q m. + autorewrite with push_Zmul zsimplify zstrip_div. + break_match; auto with lia. + Qed. + + (** In that case, we have *) + Theorem barrett_reduction_small + : a mod n = if r <? n + then r + else r - n. + Proof using a_nonneg a_small k_good m_good n_pos n_reasonable q. + pose proof r_small. pose proof qn_small. + destruct (r <? n) eqn:rlt; Z.ltb_to_lt. + { symmetry; apply (Zmod_unique a n q); subst r; lia. } + { symmetry; apply (Zmod_unique a n (q + 1)); subst r; lia. } + Qed. + End barrett_algorithm. +End barrett. |