5 files changed, 129 insertions, 18 deletions

diff --git a/INSTALL b/INSTALL
index 8ce8946..3175d25 100644
--- a/INSTALL
+++ b/INSTALL
@@ -1,10 +1,11 @@
 # Requirements
 
 A full installation of nissy requires a little more than 2Gb of space,
-of which 1.6Gb are occupied by the huge pruning table for optimal solving,
+of which 1.6Gb are occupied by the huge pruning table for fast optimal solving,
 and running it requires the same amount of RAM.
-One can choose to never use the optimal solver and not to install the relative
-pruning table. If so, about 500Mb should be enough.
+One can choose to never use this function and not to install the relative
+pruning table. There is an alternative (about 5 times slower)
+optimal solving function that uses about 500Mb of RAM.
 
 # Installation
 
diff --git a/README.md b/README.md
index 4ddbe66..89a9c80 100644
--- a/README.md
+++ b/README.md
@@ -20,11 +20,12 @@ solutions for EO/DR/HTR or similar substeps.
 
 ## Requirements
 
-A full installation of nissy requires about 1.8Gb of space, of which 1.6Gb are
-occupied by the huge pruning table for optimal solving, and running it requires
-the same amount of RAM.
-One can choose to never use the optimal solver and not to install the relative
-pruning table. If so, about 200Mb should be enough.
+A full installation of nissy requires a little more than 2Gb of space,
+of which 1.6Gb are occupied by the huge pruning table for fast optimal solving,
+and running it requires the same amount of RAM.
+One can choose to never use this function and not to install the relative
+pruning table. There is an alternative (about 5 times slower)
+optimal solving function that uses about 500Mb of RAM.
 
 ## Installation
 
diff --git a/TODO.md b/TODO.md
index 1c1ad57..98f4911 100644
--- a/TODO.md
+++ b/TODO.md
@@ -14,9 +14,6 @@ It's more of a personal reminder than anything else.
 * invert an alg
 
 ### More steps for `solve`
-* "slow" optimal solver, using drud table but all the tricks
-  of khuge (+ trick to avoid 180° moves when one of the inverse
-  probes returns exactly the target value)
 * QTM optimal solving
 * Block-building steps (cross, roux blocks, ...)
 * Other common steps (LSE, ...)
diff --git a/nissy b/nissy
index d853e7f..e3b0e0e 100755
--- a/nissy
+++ b/nissy
diff --git a/src/steps.c b/src/steps.c
index a56b47a..7478ce6 100644
--- a/src/steps.c
+++ b/src/steps.c
@@ -36,6 +36,7 @@ static int              estimate_drudfin_drud(DfsArg *arg);
 static int              estimate_htr_drud(DfsArg *arg);
 static int              estimate_htrfin_htr(DfsArg *arg);
 static int              estimate_optimal_HTM(DfsArg *arg);
+static int              estimate_light_HTM(DfsArg *arg);
 
 static bool             always_valid(Alg *alg);
 static bool             validate_singlecw_ending(Alg *alg);
@@ -55,6 +56,7 @@ static char check_drany_msg[100]   = "DR must be solved on at least one axis";
 
 /* Steps *********************************************************************/
 
+/* Optimal solvers *******************/
 Step
 optimal_HTM = {
         .shortname = "optimal",
@@ -74,6 +76,25 @@ optimal_HTM = {
         .ntables   = 2,
 };
 
+Step
+optimal_light_HTM = {
+        .shortname = "light",
+        .name      = "Optimal solve (in HTM), small table (500Mb RAM total)",
+
+        .final     = true,
+        .is_done   = is_solved,
+        .estimate  = estimate_light_HTM,
+        .ready     = check_centers,
+        .ready_msg = check_centers_msg,
+        .is_valid  = always_valid,
+        .moveset   = moveset_HTM,
+
+        .pre_trans = uf,
+
+        .tables    = {&pd_drud_sym16_HTM, &pd_corners_HTM},
+        .ntables   = 2,
+};
+
 /* EO steps **************************/
 Step
 eoany_HTM = {
@@ -813,6 +834,7 @@ htrfin_htr = {
 
 Step *steps[NSTEPS] = {
         &optimal_HTM, /* first is default */
+        &optimal_light_HTM,
 
         &eoany_HTM,
         &eofb_HTM,
@@ -1162,7 +1184,6 @@ static int
 estimate_optimal_HTM(DfsArg *arg)
 {
         int target, ret;
-        Move bl1;
         Cube aux;
 
         static const uint64_t udmask = (1<<U) | (1<<U2) | (1<<U3) |
@@ -1174,7 +1195,6 @@ estimate_optimal_HTM(DfsArg *arg)
 
         ret              = -1;
         target           = arg->d - arg->current_alg->len;
-        bl1              = base_move(arg->last1);
         arg->inverse     = (Cube){0};
         arg->badmovesinv = 0;
         arg->badmoves    = 0;
@@ -1204,17 +1224,17 @@ estimate_optimal_HTM(DfsArg *arg)
         }
 
         /* Inverse probing */
-        arg->inverse = inverse_cube(arg->cube);
-        if ((bl1 != U && bl1 != D) || (arg->ed->inverse_ud == -1)) {
-                arg->ed->inverse_ud = ptableval(&pd_khuge_HTM, arg->inverse);
+        aux = arg->inverse = inverse_cube(arg->cube);
+        if (!((1<<arg->last1) & udmask) || (arg->ed->inverse_ud == -1)) {
+                arg->ed->inverse_ud = ptableval(&pd_khuge_HTM, aux);
         }
         UPDATECHECKSTOP(ret, arg->ed->inverse_ud, target);
-        if ((bl1 != F && bl1 != B) || (arg->ed->inverse_fb == -1)) {
+        if (!((1<<arg->last1) & fbmask) || (arg->ed->inverse_fb == -1)) {
                 aux = apply_trans(fd, arg->inverse);
                 arg->ed->inverse_fb = ptableval(&pd_khuge_HTM, aux);
         }
         UPDATECHECKSTOP(ret, arg->ed->inverse_fb, target);
-        if ((bl1 != R && bl1 != L) || (arg->ed->inverse_rl == -1)) {
+        if (!((1<<arg->last1) & rlmask) || (arg->ed->inverse_rl == -1)) {
                 aux = apply_trans(rf, arg->inverse);
                 arg->ed->inverse_rl = ptableval(&pd_khuge_HTM, aux);
         }
@@ -1244,6 +1264,98 @@ estimate_optimal_HTM(DfsArg *arg)
         return arg->ed->oldret = ret;
 }
 
+static int
+estimate_light_HTM(DfsArg *arg)
+{
+        int target, ret;
+        Cube aux;
+
+        static const uint64_t udmask = (1<<U) | (1<<U2) | (1<<U3) |
+                                       (1<<D) | (1<<D2) | (1<<D3);
+        static const uint64_t rlmask = (1<<R) | (1<<R2) | (1<<R3) |
+                                       (1<<L) | (1<<L2) | (1<<L3);
+        static const uint64_t fbmask = (1<<F) | (1<<F2) | (1<<F3) |
+                                       (1<<B) | (1<<B2) | (1<<B3);
+        static const uint64_t htmask = (1<<U2) | (1<<D2) |
+                                       (1<<R2) | (1<<L2) |
+                                       (1<<F2) | (1<<B2);
+
+        ret              = -1;
+        target           = arg->d - arg->current_alg->len;
+        arg->inverse     = (Cube){0};
+        arg->badmovesinv = 0;
+        arg->badmoves    = 0;
+
+        /* Corners */
+        arg->ed->corners = ptableval(&pd_corners_HTM, arg->cube);
+        UPDATECHECKSTOP(ret, arg->ed->corners, target);
+
+        /* Normal probing */
+        arg->ed->normal_ud = ptableval(&pd_drud_sym16_HTM, arg->cube);
+        UPDATECHECKSTOP(ret, arg->ed->normal_ud, target);
+        aux = apply_trans(fd, arg->cube);
+        arg->ed->normal_fb = ptableval(&pd_drud_sym16_HTM, aux);
+        UPDATECHECKSTOP(ret, arg->ed->normal_fb, target);
+        aux = apply_trans(rf, arg->cube);
+        arg->ed->normal_rl = ptableval(&pd_drud_sym16_HTM, aux);
+        UPDATECHECKSTOP(ret, arg->ed->normal_rl, target);
+
+        /* If ret == 0, it's solved (corners + triple slice solved) */
+        if (ret == 0)
+                return 0;
+
+        /* Michel de Bondt's trick*/
+        if (arg->ed->normal_ud == arg->ed->normal_fb &&
+            arg->ed->normal_fb == arg->ed->normal_rl) {
+                UPDATECHECKSTOP(ret, arg->ed->normal_ud + 1, target);
+        }
+
+        /* Inverse probing */
+        if (!((1<<arg->last1) & htmask)) {
+                aux = arg->inverse = inverse_cube(arg->cube);
+                if (!((1<<arg->last1) & udmask) || (arg->ed->inverse_ud==-1)) {
+                        arg->ed->inverse_ud =
+                            ptableval(&pd_drud_sym16_HTM, aux);
+                }
+                UPDATECHECKSTOP(ret, arg->ed->inverse_ud, target);
+                if (!((1<<arg->last1) & fbmask) || (arg->ed->inverse_fb==-1)) {
+                        aux = apply_trans(fd, arg->inverse);
+                        arg->ed->inverse_fb =
+                            ptableval(&pd_drud_sym16_HTM, aux);
+                }
+                UPDATECHECKSTOP(ret, arg->ed->inverse_fb, target);
+                if (!((1<<arg->last1) & rlmask) || (arg->ed->inverse_rl==-1)) {
+                        aux = apply_trans(rf, arg->inverse);
+                        arg->ed->inverse_rl =
+                            ptableval(&pd_drud_sym16_HTM, aux);
+                }
+                UPDATECHECKSTOP(ret, arg->ed->inverse_rl, target);
+        }
+
+        /* Michel de Bondt's trick*/
+        if (arg->ed->inverse_ud == arg->ed->inverse_fb &&
+            arg->ed->inverse_fb == arg->ed->inverse_rl) {
+                UPDATECHECKSTOP(ret, arg->ed->inverse_ud + 1, target);
+        }
+
+        /* nxopt trick + half turn trick */
+        if (arg->ed->normal_ud == target)
+                arg->badmovesinv |= udmask | htmask;
+        if (arg->ed->normal_fb == target)
+                arg->badmovesinv |= fbmask | htmask;
+        if (arg->ed->normal_rl == target)
+                arg->badmovesinv |= rlmask | htmask;
+
+        if (arg->ed->inverse_ud == target)
+                arg->badmoves |= udmask | htmask;
+        if (arg->ed->inverse_fb == target)
+                arg->badmoves |= fbmask | htmask;
+        if (arg->ed->inverse_rl == target)
+                arg->badmoves |= rlmask | htmask;
+
+        return arg->ed->oldret = ret;
+}
+
 static bool
 always_valid(Alg *alg)
 {