From f90660bac410b8790d460be2ebd248e543a587e1 Mon Sep 17 00:00:00 2001
From: Alex Ozdemir <aozdemir@hmc.edu>
Date: Tue, 21 Jan 2020 12:15:10 -0800
Subject: Types & side-conditions for linear and affine fns (#3627)

This commit introduces types for linear combinations of arith variables
along with side conditions for their arithmetic.

It does the same for affine functions.

These primitives are ultimately used in our machinery for Farkas proofs.
---
 proofs/signatures/th_lira.plf | 91 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/proofs/signatures/th_lira.plf b/proofs/signatures/th_lira.plf
index 144239ee9..94944e39d 100644
--- a/proofs/signatures/th_lira.plf
+++ b/proofs/signatures/th_lira.plf
@@ -192,3 +192,94 @@
     (! pf_real_bound (th_holds (not (>=_IntReal t (term_int_to_real (a_int old_bound)))))
       (! sc_neg (^ (negate_linear_int_term t) neg_t)
         (th_holds (>=_IntReal neg_t (term_int_to_real (a_int neg_int_bound))))))))))))
+
+;; ======================================== ;;
+;; Linear Combinations and Affine functions ;;
+;; ======================================== ;;
+
+; Unifying type for both kinds of arithmetic variables
+(declare arith_var type)
+(declare av_from_int (! v int_var arith_var))
+(declare av_from_real (! v real_var arith_var))
+
+; Total order type -- return value for the comparison of two things
+(declare ord type)
+(declare ord_lt ord)
+(declare ord_eq ord)
+(declare ord_gt ord)
+
+; Compare two arith vars. Integers come before reals, and otherwise we use the
+; LFSC ordering
+(program arith_var_cmp ((v1 arith_var) (v2 arith_var)) ord
+  (match v1
+    ((av_from_int  i1)
+      (match v2
+        ((av_from_int  i2)
+          (ifequal i1 i2
+            ord_eq
+            (compare i1 i2 ord_lt ord_gt)))
+        ((av_from_real r2) ord_lt)))
+    ((av_from_real r1)
+      (match v2
+        ((av_from_int  i2) ord_gt)
+        ((av_from_real r2)
+          (ifequal r1 r2
+            ord_eq
+            (compare r1 r2 ord_lt ord_gt)))))))
+
+; Type for linear combinations of variables
+; NB: Functions below will assume that the list is always sorted by variable!
+(declare lc type)
+(declare lc_null lc)
+(declare lc_cons (! c mpq (! v arith_var (! rest lc lc))))
+
+; Sum of linear combinations.
+(program lc_add ((l1 lc) (l2 lc)) lc
+  (match l1
+    (lc_null l2)
+    ((lc_cons c1 v1 l1')
+      (match l2
+        (lc_null l1)
+        ((lc_cons c2 v2 l2')
+          (match (arith_var_cmp v1 v2)
+            (ord_lt (lc_cons c1 v1 (lc_add l1' l2)))
+            (ord_eq
+              (let c (mp_add c1 c2)
+                (mp_ifzero c
+                  (lc_add l1' l2')
+                  (lc_cons c v1 (lc_add l1' l2')))))
+            (ord_gt (lc_cons c2 v2 (lc_add l1 l2')))))))))
+
+; Scaling a linear combination
+(program lc_mul_c ((l lc) (c mpq)) lc
+  (match l
+    (lc_null l)
+    ((lc_cons c' v' l') (lc_cons (mp_mul c c') v' (lc_mul_c l' c)))))
+
+; Negating a linear combination
+(program lc_neg ((l lc)) lc
+         (lc_mul_c l (~ 1/1)))
+
+; An affine function of variables (a linear combination + a constant)
+(declare aff type)
+(declare aff_cons (! c mpq (! l lc aff)))
+
+; Sum of affine functions
+(program aff_add ((p1 aff) (p2 aff)) aff
+  (match p1
+    ((aff_cons c1 l1)
+      (match p2
+        ((aff_cons c2 l2) (aff_cons (mp_add c1 c2) (lc_add l1 l2)))))))
+
+; Scaling an affine function
+(program aff_mul_c ((p aff) (c mpq)) aff
+  (match p
+    ((aff_cons c' l') (aff_cons (mp_mul c' c) (lc_mul_c l' c)))))
+
+; Negating an affine function
+(program aff_neg ((p aff)) aff
+  (aff_mul_c p (~ 1/1)))
+
+; Subtracting affine functions
+(program aff_sub ((p1 aff) (p2 aff)) aff
+  (aff_add p1 (aff_neg p2)))
-- 
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