From 7c84cb433a257e25ff55b3d05a29722f09ccdf90 Mon Sep 17 00:00:00 2001
From: Simon Brooke <simon@journeyman.cc>
Date: Sun, 16 Mar 2025 09:38:00 +0000
Subject: [PATCH] Changed from using bit-shifts to using arithmetic operators.
 More tests fail, but...

---
 lisp/fact.lisp      |  2 +-
 src/arith/integer.c | 12 ++++++------
 src/arith/peano.h   |  4 +++-
 3 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/lisp/fact.lisp b/lisp/fact.lisp
index 86d452a..17a7288 100644
--- a/lisp/fact.lisp
+++ b/lisp/fact.lisp
@@ -1,6 +1,6 @@
 (set! fact
       (lambda (n)
-      "Compute the factorial of `n`, expected to be an integer."
+      "Compute the factorial of `n`, expected to be a natural number."
       (cond ((= n 1) 1)
         (t (* n (fact (- n 1)))))))
 
diff --git a/src/arith/integer.c b/src/arith/integer.c
index 0b4990a..7ca328f 100644
--- a/src/arith/integer.c
+++ b/src/arith/integer.c
@@ -71,7 +71,7 @@ struct cons_pointer make_integer( int64_t value, struct cons_pointer more ) {
 __int128_t cell_value( struct cons_pointer c, char op, bool is_first_cell ) {
     long int val = nilp( c ) ? 0 : pointer2cell( c ).payload.integer.value;
 
-    long int carry = is_first_cell ? 0 : ( MAX_INTEGER + 1 );
+    long int carry = is_first_cell ? 0 : ( INT_CELL_BASE );
 
     __int128_t result = ( __int128_t ) integerp( c ) ?
         ( val == 0 ) ? carry : val : op == '*' ? 1 : 0;
@@ -106,15 +106,15 @@ __int128_t int128_to_integer( __int128_t val,
     if ( MAX_INTEGER >= val ) {
         carry = 0;
     } else {
-        carry = val >> INTEGER_BIT_SHIFT; 
+        carry = val % INT_CELL_BASE; 
         debug_printf( DEBUG_ARITH,
                       L"int128_to_integer: 64 bit overflow; setting carry to %ld\n",
                       ( int64_t ) carry );
-        val &= MAX_INTEGER;
+        val /= INT_CELL_BASE;
     }
 
     struct cons_space_object *newc = &pointer2cell( new );
-    newc->payload.integer.value = val;
+    newc->payload.integer.value = (int64_t)val;
 
     if ( integerp( less_significant ) ) {
         struct cons_space_object *lsc = &pointer2cell( less_significant );
@@ -136,7 +136,7 @@ struct cons_pointer make_integer_128( __int128_t val,
             less_significant =
                 make_integer( ( long int ) val & MAX_INTEGER,
                               less_significant );
-            val = val >> INTEGER_BIT_SHIFT;
+            val = val * INT_CELL_BASE;
         }
 
     } while ( nilp( result ) );
@@ -361,7 +361,7 @@ struct cons_pointer integer_to_string( struct cons_pointer int_pointer,
             while ( accumulator > 0 || !nilp( next ) ) {
                 if ( accumulator < MAX_INTEGER && !nilp( next ) ) {
                     accumulator +=
-                        ( pointer2cell( next ).payload.integer.value << INTEGER_BIT_SHIFT );
+                        ( pointer2cell( next ).payload.integer.value % INT_CELL_BASE );
                     next = pointer2cell( next ).payload.integer.more;
                 }
                 int offset = ( int ) ( accumulator % base );
diff --git a/src/arith/peano.h b/src/arith/peano.h
index eb53450..95c5013 100644
--- a/src/arith/peano.h
+++ b/src/arith/peano.h
@@ -21,7 +21,9 @@
  * 
  * So left shifting and right shifting by 60 bits is correct.
  */
-#define MAX_INTEGER ((__int128_t)0x0fffffffffffffffL)
+#define MAX_INTEGER   ((__int128_t)0x0fffffffffffffffL)
+#define INT_CELL_BASE ((__int128_t)MAX_INTEGER + 1) // ((__int128_t)0x1000000000000000L)
+
 /**
  * @brief Number of value bits in an integer cell
  *