From 6b789c7331b3c955151fc6a8e286e701ed87fae3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lars=20G=C3=B6ttgens?= <lars.goettgens@rwth-aachen.de>
Date: Wed, 27 Sep 2023 17:18:01 +0200
Subject: [PATCH] Add `mul(::SMat, ::SMat)`

---
 docs/src/sparse/intro.md |  1 +
 src/Sparse/Matrix.jl     | 19 +++++++++++++++++--
 test/Sparse/Matrix.jl    |  5 +++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/docs/src/sparse/intro.md b/docs/src/sparse/intro.md
index 6015c9cb1e..d05289b07c 100644
--- a/docs/src/sparse/intro.md
+++ b/docs/src/sparse/intro.md
@@ -209,6 +209,7 @@ Hecke.mul(::SMat{T}, ::AbstractVector{T}) where {T}
 Hecke.mul(::SMat{T}, ::AbstractMatrix{T})  where {T}
 Hecke.mul(::SMat{T}, ::MatElem{T}) where {T}
 Hecke.mul(::SRow{T}, ::SMat{T}) where {T}
+Hecke.mul(::SMat{T}, ::SMat{T}) where {T}
 ```
 
 Other:
diff --git a/src/Sparse/Matrix.jl b/src/Sparse/Matrix.jl
index c510ebaa63..70a75f2949 100644
--- a/src/Sparse/Matrix.jl
+++ b/src/Sparse/Matrix.jl
@@ -570,7 +570,6 @@ function mul!(c::MatElem{T}, A::SMat{T}, b::MatElem{T}) where T
 end
 
 # - SMat{T} * MatElem{T} as MatElem{T}
-
 @doc raw"""
     mul(A::SMat{T}, b::MatElem{T}) -> MatElem
 
@@ -583,7 +582,6 @@ function mul(A::SMat{T}, b::MatElem{T}) where T
 end
 
 # - SRow{T} * SMat{T} as SRow{T}
-
 @doc raw"""
     mul(A::SRow, B::SMat) -> SRow
 
@@ -600,6 +598,23 @@ function mul(A::SRow{T}, B::SMat{T}) where T
   return C
 end
 
+# - SMat{T} * SMat{T} as SMat{T}
+@doc raw"""
+    mul(A::SMat, B::SMat) -> SMat
+
+Return the product $A\cdot B$ as a sparse matrix.
+"""
+function mul(A::SMat{T}, B::SMat{T}) where {T}
+  @req ncols(A) == nrows(B) "Matrices must have compatible dimensions"
+  @req base_ring(A) == base_ring(B) "Matrices must have same base ring"
+  C = sparse_matrix(base_ring(B), nrows(A), ncols(B))
+  for (i, rA) in enumerate(A.rows)
+    rC = mul(rA, B)
+    C[i] = rC
+  end
+  return C
+end
+
 ################################################################################
 #
 #  Multiplication with reduction
diff --git a/test/Sparse/Matrix.jl b/test/Sparse/Matrix.jl
index a21589269c..a8c1985965 100644
--- a/test/Sparse/Matrix.jl
+++ b/test/Sparse/Matrix.jl
@@ -156,6 +156,11 @@ using Hecke.SparseArrays
   w = @inferred mul(v, D)
   @test w == sparse_row(FlintZZ)
 
+  D1 = sparse_matrix(FlintZZ, [81 0 2; 31 0 -5])
+  D2 = sparse_matrix(FlintZZ, [12 403 -23; 0 0 122; -1 2 99])
+  E = @inferred mul(D1, D2)
+  @test E == sparse_matrix(matrix(D1) * matrix(D2))
+
   # Addition
 
   D = sparse_matrix(FlintZZ, [1 5 3; 0 0 0; 0 1 0])