From e6c975d48f918856396ce4ae6824b9c387930f3f Mon Sep 17 00:00:00 2001
From: Anton Khirnov <anton@khirnov.net>
Date: Mon, 20 Jan 2020 15:46:53 +0100
Subject: mg2d: export the residual norm to the caller

---
 mg2d.c |  1 +
 mg2d.h | 10 +++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/mg2d.c b/mg2d.c
index b669569..95bd1ee 100644
--- a/mg2d.c
+++ b/mg2d.c
@@ -1155,6 +1155,7 @@ int mg2d_solve(MG2DContext *ctx)
 fail:
     mg2di_log(&priv->logger, MG2D_LOG_ERROR, "The solver failed to converge\n");
 finish:
+    ctx->residual_max = s_root->residual_max;
     mg2di_timer_stop(&priv->timer_solve);
     return ret;
 }
diff --git a/mg2d.h b/mg2d.h
index 01698a8..8f48361 100644
--- a/mg2d.h
+++ b/mg2d.h
@@ -239,6 +239,13 @@ typedef struct MG2DContext {
      * Set by mg2d_solver_alloc[_mpi]().
      */
     size_t    local_size[2];
+
+    /**
+     * Maximum of the absolute value of the residual.
+     *
+     * Set by mg2d_solve() if it returns 0 or MG2D_ERR_MAXITER_REACHED.
+     */
+    double residual_max;
 } MG2DContext;
 
 /**
@@ -271,7 +278,8 @@ MG2DContext *mg2d_solver_alloc_mpi(MPI_Comm comm, const size_t local_start[2],
  *   - 0 on success
  *   - MG2D_ERR_MAXITER_REACHED if desired tolerance was not reached after
  *     maximum allowed number of iterations were performed. The final
- *     value of the solution is still exported in ctx->u.
+ *     value of the solution is still exported in ctx->u, ctx->residual_max is
+ *     set to the final residual norm.
  *   - MG2D_ERR_DIVERGE if the iteration process has diverged.
  *   - another negative error code on other types of failure
  */
-- 
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