Subtraction seems to work now.

implementations/bigints.joy

@@ -1,34 +1,93 @@
-base 2147483648
-ditch-empty-list [bool] [popd] [pop] ifte
-bool_to_int [0] [1] branch
-uncons-two [uncons] ii swapd
+clear
+[base 2147483648]
+[ditch-empty-list [bool] [popd] [pop] ifte]
+[bool-to-int [0] [1] branch]
+[uncons-two [uncons] ii swapd]
+[sandwich swap [cons] dip swoncat]
 
-add-with-carry _add-with-carry0 _add-with-carry1
-_add-with-carry0 [bool_to_int] dipd + +
-_add-with-carry1 base [mod] [>=] clop
+[digitalize [0 <=] [pop []] [base divmod swap] [i cons] genrec]
+[to-bigint [!-] [abs digitalize] cleave cons]
 
-add-carry-to-digits [_actd_P] [_actd_THEN] [_actd_R0] [_actd_R1] genrec
-_actd_P pop not
-_actd_THEN popd
-_actd_R0 [bool] [_actd_R0.then] [_actd_R0.else] ifte
-_actd_R0.else popd 1 false rolldown
-_actd_R0.then 0 swap uncons [add-with-carry] dip
-_actd_R1 i cons
+[prep rest 1 0 rolldown]
+[from-bigint' [next-digit] step popd]
+[next-digit [increase-power] [accumulate-digit] clop popdd]
+[increase-power popop base *]
+[accumulate-digit rolldown * +]
 
-add-digits initial-carry add-digits'
-initial-carry false rollup
-add-digits' [P] [THEN] [R0] [R1] genrec
-P [bool] ii & not
-THEN [P'] [THEN'] [ELSE] ifte
-R0 uncons-two [add-with-carry] dipd
-R1 i cons
-P' [bool] ii |
-THEN' ditch-empty-list add-carry-to-digits
-ELSE pop swap [] [1 swons] branch
+[sign-int [first] [prep from-bigint'] cleave]
+[neg-if-necessary swap [neg] [] branch]
+[from-bigint sign-int neg-if-necessary]
+
+[add-with-carry _add-with-carry0 _add-with-carry1]
+[_add-with-carry0 [bool-to-int] dipd + +]
+[_add-with-carry1 base [mod] [>=] clop]
+
+[add-carry-to-digits [pop not] [popd] [_actd_R0] [i cons] genrec]
+[_actd_R0 [bool] [_actd_R0.then] [_actd_R0.else] ifte]
+[_actd_R0.else popd 1 false rolldown]
+[_actd_R0.then 0 swap uncons [add-with-carry] dip]
+
+[add-digits initial-carry add-digits']
+[initial-carry false rollup]
+
+[add-digits' [P] [THEN] [R0] [R1] genrec]
+[P [bool] ii & not]
+[THEN [P'] [THEN'] [ELSE] ifte]
+[R0 uncons-two [add-with-carry] dipd]
+[R1 i cons]
+[P' [bool] ii |]
+[THEN' ditch-empty-list add-carry-to-digits]
+[ELSE pop swap [] [1 swons] branch]
+
+[same-sign [first] ii xor not]
+[add-like-bigints [uncons] dip rest add-digits cons]
+[add-bigints [same-sign] [add-like-bigints] [1 0 /] ifte]
+
+[build-two-list-combiner _btlc0 _btlc1 [i cons]]
+[_btlc0.0 [[ditch-empty-list] swoncat] dip]
+[_btlc0.1 [pop] swoncat]
+[_btlc0.3 [_btlc0.0 _btlc0.1] dip]
+[_btlc0.4 [uncons-two] [dipd] sandwich]
+[_btlc0 _btlc0.3 _btlc0.4]
+[_btlc1 [[ifte] ccons [P'] swons [P] swap] dip]
+
+[carry [] [1 swons] branch]
+
+[compare-pairs [bool not] [pop false] [[first [>=] infrst] [pop true]] [[rest] swoncat ifte] genrec]
+[xR1 uncons-two [unit cons swons] dipd]
+[xP [bool] ii & not]
+[BASE [bool] [popop pop true] [[pop bool] [popop pop false] [popop compare-pairs] ifte] ifte]
+[gt-bigint <<{} [xP] [BASE] [xR1] tailrec]
+[check-gt [gt-bigint] [swap [not] dipd] [] ifte]
+
+[sub-carry pop]
+
+[sub-carry-from-digits popd]
+[_sub-with-carry0 rolldown bool-to-int [-] ii]
+[_sub-with-carry1 [base + base mod] [0 <] cleave]
+[sub-with-carry _sub-with-carry0 _sub-with-carry1]
+
+[sub-like-bigints [uncons] dip rest check-gt sub-digits cons]
+[sub-digits initial-carry sub-digits']
+
+enstacken [inscribe] step
+
+[add-carry-to-digits]
+[swap carry]
+[add-with-carry]
+build-two-list-combiner
+[genrec] ccons ccons
+[add-digits'] swoncat
+inscribe
+
+[sub-carry-from-digits]
+[swap sub-carry]
+[sub-with-carry]
+build-two-list-combiner
+[genrec] ccons ccons
+[sub-digits'] swoncat
+inscribe
 
 
-add-bigints 
-[[first] ii =]                       # are they the same sign?
-[[uncons] dip rest add-digits cons]  # add the digits and set the sign.
-[neg-bigint sub-bigints]             # adding unlikes is actually subtraction.
-ifte
+
+[true 456] [true 123] sub-like-bigints