read and write optimal LP basis in gurobi and Cbc

Author: h-g-s

mip/cbc.py

@@ -141,6 +141,11 @@
 
     void Cbc_readLp(Cbc_Model *model, const char *file);
 
+    int Cbc_readBasis(Cbc_Model *model, const char *filename);
+
+    int Cbc_writeBasis(Cbc_Model *model, const char *filename, char
+        writeValues, int formatType);
+
     void Cbc_readMps(Cbc_Model *model, const char *file);
 
     char Cbc_supportsGzip();
@@ -1441,9 +1446,11 @@ def write(self, file_path: str):
             cbclib.Cbc_writeMps(self._model, fpstr)
         elif ".lp" in file_path.lower():
             cbclib.Cbc_writeLp(self._model, fpstr)
+        elif ".bas" in file_path.lower():
+            cbclib.Cbc_writeBasis(self._model, fpstr, CHAR_ONE, 2)
         else:
             raise ValueError(
-                "Enter a valid extension (.lp or .mps) \
+                "Enter a valid extension (.lp, .mps or .bas) \
                 to indicate the file format"
             )
 
@@ -1464,9 +1471,18 @@ def read(self, file_path: str) -> None:
             cbclib.Cbc_readMps(self._model, fpstr)
         elif ".lp" in file_path.lower():
             cbclib.Cbc_readLp(self._model, fpstr)
+        elif ".bas" in file_path.lower():
+            status = cbclib.Cbc_readBasis(self._model, fpstr)
+            if status == -1:
+                raise IOError("Error reading %s" % file_path)
+            elif status == 0:
+                logger.warning("No values read from %s" % file_path)
+            elif status == 1:
+                logger.info("Optimal LP basis successfully loaded.")
+
         else:
             raise ValueError(
-                "Enter a valid extension (.lp or .mps) \
+                "Enter a valid extension (.lp, .mps or .bas) \
                 to indicate the file format"
             )
 

mip/gurobi.py

@@ -216,6 +216,8 @@
 
     int GRBreadmodel(GRBenv *env, const char *filename, GRBmodel **modelP);
 
+    int GRBread(GRBmodel *model, const char *filename);
+
     int GRBdelvars(GRBmodel *model, int numdel, int *ind );
 
     int GRBsetcharattrlist(GRBmodel *model, const char *attrname,
@@ -258,6 +260,7 @@
 GRBsetdblattrlist = grblib.GRBsetdblattrlist
 GRBwrite = grblib.GRBwrite
 GRBreadmodel = grblib.GRBreadmodel
+GRBread = grblib.GRBread
 GRBgetconstrbyname = grblib.GRBgetconstrbyname
 GRBupdatemodel = grblib.GRBupdatemodel
 GRBgetcharattrelement = grblib.GRBgetcharattrelement
@@ -916,14 +919,29 @@ def write(self, file_path: str) -> None:
     def read(self, file_path: str) -> None:
         if not isfile(file_path):
             raise FileNotFoundError("File {} does not exist".format(file_path))
-        GRBfreemodel(self._model)
-        self._model = ffi.new("GRBmodel **")
-        st = GRBreadmodel(self._env, file_path.encode("utf-8"), self._model)
-        if st != 0:
-            raise InterfacingError(
-                "Could not read model {}, check contents".format(file_path)
-            )
-        self._model = self._model[0]
+
+        lfile = file_path.lower()
+
+        MEXTS = [".mps.gz", ".mps.bz2", ".lp.gz", ".lp.bz2", ".lp", ".mps"]
+        is_model = False
+        for ext in MEXTS:
+            if lfile.endswith(ext):
+                is_model = True
+                break
+
+        if is_model:
+            GRBfreemodel(self._model)
+            self._model = ffi.new("GRBmodel **")
+            st = GRBreadmodel(self._env, file_path.encode("utf-8"), self._model)
+            if st != 0:
+                raise InterfacingError(
+                    "Could not read model {}, check contents".format(file_path)
+                )
+            self._model = self._model[0]
+        else:
+            error = GRBread(self._model, file_path.encode("utf-8"))
+            if error:
+                raise IOError("Error loading %s" % file_path)
 
     def num_cols(self) -> int:
         return self.get_int_attr("NumVars") + self.__n_cols_buffer

mip/libraries/cbc-c-darwin-x86-64.dylib

@@ -645,7 +645,10 @@ def read(self: "Model", path: str):
              `MPS file format <https://en.wikipedia.org/wiki/MPS_(format)>`_
 
            :code:`.sol`
-             initial feasible solution
+             initial integer feasible solution
+
+           :code:`.bas`
+             `optimal basis <http://lpsolve.sourceforge.net/5.5/bas-format.htm>`_ for the linear programming relaxation.
 
         Note: if a new problem is readed, all variables, constraints
         and parameters from the current model will be cleared.
@@ -681,6 +684,14 @@ def read(self: "Model", path: str):
             self.start = var_list
             return
 
+        if path.lower().endswith(".bas"):
+            if self.num_cols == 0:
+                raise mip.ProgrammingError(
+                    "Cannot load optimal LP basis for empty model."
+                )
+            self.solver.read(path)
+            return
+
         # reading model
         model_ext = [".lp", ".mps", ".mps.gz"]
 
@@ -723,6 +734,9 @@ def write(self: "Model", file_path: str):
            :code:`.sol`
              initial feasible solution
 
+           :code:`.bas`
+             `optimal basis <http://lpsolve.sourceforge.net/5.5/bas-format.htm>`_ for the linear programming relaxation.
+
         Args:
             file_path(str): file name
         """
@@ -732,7 +746,11 @@ def write(self: "Model", file_path: str):
             else:
                 mip_start = [(var, var.x) for var in self.vars if abs(var.x) >= 1e-8]
                 save_mipstart(mip_start, file_path)
-        elif file_path.lower().endswith(".lp") or file_path.lower().endswith(".mps"):
+        elif (
+            file_path.lower().endswith(".lp")
+            or file_path.lower().endswith(".mps")
+            or file_path.lower().endswith(".bas")
+        ):
             self.solver.write(file_path)
         else:
             raise ValueError(