1 (* ========================================================================= *)
2 (* Euclidean GCD algorithm. *)
3 (* ========================================================================= *)
5 needs "Library/prime.ml";;
8 `egcd(m,n) = if m = 0 then n
10 else if m <= n then egcd(m,n - m)
13 (* ------------------------------------------------------------------------- *)
15 (* ------------------------------------------------------------------------- *)
17 let EGCD_INVARIANT = prove
18 (`!m n d. d divides egcd(m,n) <=> d divides m /\ d divides n`,
19 GEN_TAC THEN GEN_TAC THEN WF_INDUCT_TAC `m + n` THEN
20 GEN_TAC THEN ONCE_REWRITE_TAC[egcd] THEN
21 ASM_CASES_TAC `m = 0` THEN ASM_REWRITE_TAC[DIVIDES_0] THEN
22 ASM_CASES_TAC `n = 0` THEN ASM_REWRITE_TAC[DIVIDES_0] THEN
24 (W(fun (asl,w) -> FIRST_X_ASSUM(fun th ->
25 MP_TAC(PART_MATCH (lhs o snd o dest_forall o rand) th (lhand w)))) THEN
26 ANTS_TAC THENL [ASM_ARITH_TAC; ALL_TAC]) THEN
27 ASM_MESON_TAC[DIVIDES_SUB; DIVIDES_ADD; SUB_ADD; LE_CASES]);;
29 (* ------------------------------------------------------------------------- *)
30 (* Hence we get the proper behaviour, and it's equal to the real GCD. *)
31 (* ------------------------------------------------------------------------- *)
34 (`!m n. egcd(m,n) = gcd(m,n)`,
35 ONCE_REWRITE_TAC[GSYM GCD_UNIQUE] THEN
36 MESON_TAC[EGCD_INVARIANT; DIVIDES_REFL]);;
39 (`!a b. (egcd (a,b) divides a /\ egcd (a,b) divides b) /\
40 (!e. e divides a /\ e divides b ==> e divides egcd (a,b))`,
41 REWRITE_TAC[EGCD_GCD; GCD]);;