Merge pull request fortran-lang#1158 from srinjoy933/feat-csc2dense2

Enhancement: Add csc2dense conversion routine to complete dense API

Author: jvdp1

doc/specs/stdlib_sparse.md

@@ -296,6 +296,16 @@ If the `diagonal` array has not been previously allocated, the `diag` subroutine
 
 ### Syntax
 
+`call ` [[stdlib_sparse_conversion(module):csc2dense(interface)]] `(csc,dense)`
+
+### Arguments
+
+`csc` : Shall be a `CSC` type of `real` or `complex` type. It is an `intent(in)` argument.
+
+`dense` : Shall be a rank-2 array of `real` or `complex` type. It is an `intent(out)` argument.
+
+### Syntax
+
 `call ` [[stdlib_sparse_conversion(module):coo2csr(interface)]] `(coo,csr[,sort_data])`
 
 ### Arguments

src/sparse/stdlib_sparse_conversion.fypp

@@ -85,6 +85,19 @@ module stdlib_sparse_conversion
         #:endfor
     end interface
     public :: csr2dense
+    
+    !! version: experimental
+    !!
+    !! Conversion from csc to dense
+    !! Enables creating a dense 2D matrix from the non-zero values stored in a CSC format
+    !! The dense matrix can be allocated on the fly if not pre-allocated by the user.
+    !! [Specifications](../page/specs/stdlib_sparse.html#sparse_conversion)
+    interface csc2dense
+        #:for k1, t1, s1 in (KINDS_TYPES)
+        module procedure :: csc2dense_${s1}$
+        #:endfor
+    end interface
+    public :: csc2dense
 
     !! version: experimental
     !!
@@ -317,6 +330,32 @@ contains
             end do
         end if
     end subroutine
+    
+    #:endfor
+
+    #:for k1, t1, s1 in (KINDS_TYPES)
+    subroutine csc2dense_${s1}$(CSC,dense)
+        type(CSC_${s1}$_type), intent(in) :: CSC
+        ${t1}$, allocatable, intent(out) :: dense(:,:)
+        integer(ilp) :: i, j
+
+        if(.not.allocated(dense)) allocate(dense(CSC%nrows,CSC%ncols),source=zero_${s1}$)
+        if( CSC%storage == sparse_full) then
+            do j = 1, CSC%ncols
+                do i = CSC%colptr(j), CSC%colptr(j+1)-1
+                    dense(CSC%row(i),j) = CSC%data(i)
+                end do
+            end do
+        else
+            do j = 1, CSC%ncols
+                do i = CSC%colptr(j), CSC%colptr(j+1)-1
+                    dense(CSC%row(i),j) = CSC%data(i)
+                    if( j == CSC%row(i) ) cycle
+                    dense(j,CSC%row(i)) = CSC%data(i)
+                end do
+            end do
+        end if
+    end subroutine
 
     #:endfor
 

test/linalg/test_linalg_sparse.fypp

@@ -13,7 +13,6 @@ module test_sparse_spmv
 
 contains
 
-
     !> Collect all exported unit tests
     subroutine collect_suite(testsuite)
         !> Collection of tests
@@ -30,7 +29,8 @@ contains
             new_unittest('diagonal', test_diagonal), &
             new_unittest('add_get_values', test_add_get_values),  &
             new_unittest('sparse_operators', test_sparse_operators), &
-            new_unittest('add_block_symmetric_skip', test_add_block_symmetric_skip) &
+            new_unittest('add_block_symmetric_skip', test_add_block_symmetric_skip), &
+            new_unittest('csc2dense', test_csc2dense) &
         ]
     end subroutine
 
@@ -579,10 +579,40 @@ contains
         #:endfor
 
     end subroutine
+
+    subroutine test_csc2dense(error)
+        !> Error handling
+        type(error_type), allocatable, intent(out) :: error
+        #:for k1, t1, s1 in (KINDS_TYPES)
+        block
+            integer, parameter :: wp = ${k1}$
+            type(COO_${s1}$_type) :: COO
+            type(CSC_${s1}$_type) :: CSC
+            ${t1}$, allocatable :: dense_original(:,:), dense_converted(:,:)
+
+            ! 1. Create a baseline dense 4x4 matrix
+            allocate( dense_original(4,4) , source = &
+                    reshape(real([1,0,0,5, &
+                                  0,2,0,0, &
+                                  0,6,3,0, &
+                                  0,0,7,4],kind=wp),[4,4]) )
+
+            ! 2. Convert Dense -> COO -> CSC (Standard pipeline)
+            call dense2coo( dense_original , COO )
+            call coo2csc( COO, CSC )
+
+            ! 3. Execute the csc2dense conversion
+            call csc2dense( CSC, dense_converted )
+
+            ! 4. Verify the converted matrix perfectly matches the original
+            call check(error, all(dense_original == dense_converted), "Error: csc2dense conversion failed for kind ${s1}$")
+            if (allocated(error)) return
+        end block
+        #:endfor
+    end subroutine
 
 end module
 
-
 program tester
     use, intrinsic :: iso_fortran_env, only : error_unit
     use testdrive, only : run_testsuite, new_testsuite, testsuite_type
@@ -607,4 +637,4 @@ program tester
         write(error_unit, '(i0, 1x, a)') stat, "test(s) failed!"
         error stop
     end if
-end program
+end program