From 45f41504f74896cf91733878fb5f012af653d967 Mon Sep 17 00:00:00 2001
From: Anton Khirnov <anton@khirnov.net>
Date: Sun, 3 Mar 2019 14:10:04 +0100
Subject: mg2d: fail if the residual grows too much from the original one

---
 mg2d.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mg2d.c b/mg2d.c
index 5b94022..a8511da 100644
--- a/mg2d.c
+++ b/mg2d.c
@@ -512,7 +512,7 @@ int mg2d_solve(MG2DContext *ctx)
     MG2DLevel    *root = priv->root;
     EGSContext *s_root = root->solver;
     int64_t time_start;
-    double res_prev, res_cur;
+    double res_orig, res_prev, res_cur;
     int ret;
 
     if (!priv->tp) {
@@ -531,7 +531,8 @@ int mg2d_solve(MG2DContext *ctx)
     if (ret < 0)
         return ret;
 
-    res_prev = s_root->residual_max;
+    res_orig = s_root->residual_max;
+    res_prev = res_orig;
 
     for (int i = 0; i < ctx->maxiter; i++) {
         ret = mg_solve_subgrid(ctx, root);
@@ -553,7 +554,7 @@ int mg2d_solve(MG2DContext *ctx)
         mg2di_log(&priv->logger, MG2D_LOG_VERBOSE,
                   "finished toplevel iteration %d, residual %g -> %g (%g)\n",
                   i, res_prev, res_cur, res_prev / res_cur);
-        if (res_cur / res_prev > 1e1) {
+        if (res_cur / res_prev > 1e1 || res_cur / res_orig > 1e3) {
             mg2di_log(&priv->logger, MG2D_LOG_ERROR, "A multigrid iteration diverged\n");
             ret = MG2D_ERR_DIVERGE;
             goto fail;
-- 
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