commerce: discount-aware (net) tax policy (11 tests) — Phase 5 ext

nettax.sx — alternative to quote.sx's gross-tax default: cart-quote-net taxes
the net (post-discount) base. allocate-discount spreads the basket discount
across lines by extended-price share with a deterministic largest-remainder
pass so per-line shares sum exactly to the discount; each line taxed on its net
at its class rate. Both policies reproducible; pick per jurisdiction.
Total 239/239 across 15 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/commerce/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window)
+SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window nettax)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -67,6 +67,7 @@ run_suite() {
 (load "lib/commerce/stack.sx")
 (load "lib/commerce/quote.sx")
 (load "lib/commerce/window.sx")
+(load "lib/commerce/nettax.sx")
 (load "lib/commerce/ledger.sx")
 (load "lib/commerce/order.sx")
 (load "lib/commerce/payment.sx")

lib/commerce/nettax.sx

@@ -0,0 +1,80 @@
+;; lib/commerce/nettax.sx — discount-aware tax (alternative policy).
+;;
+;; price.sx / quote.sx tax the GROSS per-line amounts (discount reduces payable
+;; but not the tax base). This module is the alternative explicit policy: tax the
+;; NET (post-discount) base. The basket-level discount is allocated across lines
+;; in proportion to each line's extended price, with a deterministic
+;; largest-remainder pass so per-line shares sum EXACTLY to the discount; tax is
+;; then charged on each line's net at its class rate.
+;;
+;; Both policies are reproducible from (ctx, cart, ruleset, exclusions); pick the
+;; one the jurisdiction requires. cart-quote-net mirrors cart-quote's shape.
+
+(define ct-sum (fn (xs) (reduce (fn (a x) (+ a x)) 0 xs)))
+
+;; Add 1 to the first `rem` elements (deterministic remainder distribution).
+(define
+  ct-add-rem
+  (fn
+    (xs rem)
+    (cond
+      ((empty? xs) (list))
+      ((> rem 0)
+        (cons
+          (+ (first xs) 1)
+          (ct-add-rem (rest xs) (- rem 1))))
+      (:else xs))))
+
+;; Per-line discount allocation (parallel to cart), summing exactly to
+;; total-discount, proportional to line-extended share.
+(define
+  allocate-discount
+  (fn
+    (cat cart total-discount)
+    (let
+      ((sub (cart-subtotal cat cart)))
+      (if
+        (= sub 0)
+        (map (fn (l) 0) cart)
+        (let
+          ((floors (map (fn (l) (quotient (* total-discount (line-extended cat l)) sub)) cart)))
+          (ct-add-rem floors (- total-discount (ct-sum floors))))))))
+
+;; Tax on one line's net (extended - allocated discount), clamped at 0.
+(define
+  net-line-tax
+  (fn
+    (ctx line alloc)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((net (- (line-extended cat line) alloc)))
+        (apply-bps
+          (if (< net 0) 0 net)
+          (rate-bps
+            (get ctx :tax-rules)
+            (get ctx :jurisdiction)
+            (catalog-class cat (line-sku line))
+            (get ctx :customer)))))))
+
+(define
+  net-tax
+  (fn
+    (ctx cart allocations)
+    (ct-sum
+      (map (fn (line alloc) (net-line-tax ctx line alloc)) cart allocations))))
+
+;; Discount-aware quote: tax computed on the net (post-discount) base.
+(define
+  cart-quote-net
+  (fn
+    (ctx cart ruleset exclusions)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((sub (cart-subtotal cat cart))
+          (disc (best-promo-discount ctx cart ruleset exclusions))
+          (codes (best-promo-codes ctx cart ruleset exclusions)))
+        (let
+          ((tax (net-tax ctx cart (allocate-discount cat cart disc))))
+          {:codes codes :subtotal sub :discount disc :total (+ (- sub disc) tax) :tax tax})))))

lib/commerce/scoreboard.json

@@ -13,9 +13,10 @@
     "federation": {"pass": 12, "fail": 0},
     "attribution": {"pass": 16, "fail": 0},
     "payment": {"pass": 7, "fail": 0},
-    "window": {"pass": 19, "fail": 0}
+    "window": {"pass": 19, "fail": 0},
+    "nettax": {"pass": 11, "fail": 0}
   },
-  "total_pass": 228,
+  "total_pass": 239,
   "total_fail": 0,
-  "total": 228
+  "total": 239
 }

lib/commerce/scoreboard.md

@@ -18,4 +18,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | attribution | 16 | 0 | 16 |
 | payment | 7 | 0 | 7 |
 | window | 19 | 0 | 19 |
-| **Total** | **228** | **0** | **228** |
+| nettax | 11 | 0 | 11 |
+| **Total** | **239** | **0** | **239** |

lib/commerce/tests/nettax.sx

@@ -0,0 +1,92 @@
+;; lib/commerce/tests/nettax.sx — discount-aware (net) tax policy.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+(define
+  pcat
+  (make-catalog
+    (list
+      (list "widget" 1000 :standard)
+      (list "tea" 1000 :reduced))
+    (list)
+    (list)))
+
+(define
+  rules
+  (list
+    (list :uk :standard :guest 2000)
+    (list :uk :reduced :guest 500)))
+
+(define gctx (make-pricing-context pcat rules :uk :guest))
+
+;; widget x3 = 3000 (standard), tea x6 = 6000 (reduced); subtotal 9000
+(define
+  cart
+  (list (list "widget" :none 3) (list "tea" :none 6)))
+
+(define ruleset (list (list :percent "TEN" :standard 1000)))
+
+;; --- allocation: proportional, sums exactly to the discount ---
+
+(commerce-test
+  "allocate-even"
+  (allocate-discount pcat cart 300)
+  (list 100 200))
+(commerce-test
+  "allocate-sums-to-discount"
+  (ct-sum (allocate-discount pcat cart 300))
+  300)
+
+;; remainder distribution: 100 over (3000,6000)/9000 = (33,66) rem 1 -> (34,66)
+(commerce-test
+  "allocate-remainder"
+  (allocate-discount pcat cart 100)
+  (list 34 66))
+(commerce-test
+  "allocate-remainder-sums"
+  (ct-sum (allocate-discount pcat cart 100))
+  100)
+
+(commerce-test
+  "allocate-zero"
+  (allocate-discount pcat cart 0)
+  (list 0 0))
+(commerce-test
+  "allocate-empty"
+  (allocate-discount pcat empty-cart 0)
+  (list))
+
+;; --- net tax vs gross tax ---
+;; discount = TEN 10% of standard 3000 = 300, allocated (100 200).
+;; net: widget 2900@20%=580, tea 5800@5%=290 -> net tax 870 (gross was 900).
+
+(commerce-test
+  "net-quote"
+  (cart-quote-net gctx cart ruleset (list))
+  {:codes (list "TEN") :subtotal 9000 :discount 300 :total 9570 :tax 870})
+
+;; same cart through the gross policy taxes 900 (the documented default)
+(commerce-test
+  "gross-quote-for-contrast"
+  (quote-tax (cart-quote gctx cart ruleset (list)))
+  900)
+
+(commerce-test
+  "net-tax-lower"
+  (quote-tax (cart-quote-net gctx cart ruleset (list)))
+  870)
+
+;; --- no discount: net policy == gross policy ---
+
+(commerce-test
+  "no-discount-net-equals-gross"
+  (=
+    (cart-quote-net gctx cart (list) (list))
+    (cart-quote gctx cart (list) (list)))
+  true)
+
+;; --- empty cart ---
+
+(commerce-test
+  "net-empty"
+  (cart-quote-net gctx empty-cart ruleset (list))
+  {:codes (list) :subtotal 0 :discount 0 :total 0 :tax 0})

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **228/228** (14 suites; + window) — **roadmap complete; Phase 5 extensions in progress**
+`bash lib/commerce/conformance.sh` → **239/239** (15 suites; + nettax) — **roadmap complete; Phase 5 extensions in progress**
 
 ## Ground rules
 
@@ -84,8 +84,9 @@ that unlocks the most tests per effort each iteration.
 - [x] time-windowed promotions — `window.sx`: windowed promo `(promo from until)`,
   `active-ruleset`/`active-codes`/`windowed-quote` gate by datetime; feeds the
   existing promo/stack/quote pipeline unchanged. Determinism preserved.
-- [ ] discount-aware tax policy — alternative `cart-quote` computing tax on the
-  net (post-discount) base via proportional class allocation; explicit + tested.
+- [x] discount-aware tax policy — `nettax.sx`: `cart-quote-net` taxes the net
+  (post-discount) base; `allocate-discount` spreads the basket discount across
+  lines by extended share with largest-remainder so per-line shares sum exactly.
 - [ ] refund as a flow — refund lifecycle (request → approve → settle) as a second
   flow-on-sx flow, recorded in the ledger; idempotent.
 - [ ] stock-constrained reservation — order-begin! fails (railway `fail`) when
@@ -97,6 +98,13 @@ that unlocks the most tests per effort each iteration.
   agnostic; `order-settle!(ref, amount)` is the resume seam.
 
 ## Progress log
+- 2026-06-07 — `nettax.sx` (Phase 5 ext): discount-aware tax — the alternative to
+  quote.sx's gross-tax default. `cart-quote-net` taxes the NET (post-discount)
+  base. `allocate-discount` spreads the basket-level discount across lines in
+  proportion to extended price with a deterministic largest-remainder pass so
+  per-line shares sum EXACTLY to the discount; each line is then taxed on its net
+  at its class rate. Both policies reproducible from inputs; pick per jurisdiction.
+  nettax suite 11/11; total 239/239 (15 suites).
 - 2026-06-07 — `window.sx` (Phase 5 ext): time-windowed promotions. A validity
   window is kept SEPARATE from the promo tuple — windowed promo `(promo from
   until)` (inclusive int timestamps, nil = open bound). `active-ruleset` filters