altos: seek forward on FAT cluster chain instead of restarting

This improves sequential file performance by taking advantage of any
previous cached cluster/offset pair and starting from there when the
cluster changes rather than starting from scratch at the begining again.

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/src/drivers/ao_fat.c b/src/drivers/ao_fat.c
index 65c5ea7cb6f449da95b68ff9bb7f242b9bb5d4c1..98f57d674c6c3e3cd647422b65972d6db9db8885 100644 (file)
--- a/src/drivers/ao_fat.c
+++ b/src/drivers/ao_fat.c
@@ -375,10 +375,20 @@ ao_fat_current_sector(void)
 
        sector_offset = ao_file_offset >> SECTOR_SHIFT;
 
-       if (!ao_file_cluster) {
+       if (!ao_file_cluster || ao_file_offset < ao_file_cluster_offset) {
+               ao_file_cluster = ao_file_dirent.cluster;
+               ao_file_cluster_offset = 0;
+       }
+
+       if (ao_file_cluster_offset + bytes_per_cluster <= ao_file_offset) {
+               uint16_t        cluster_distance;
+
                cluster_offset = sector_offset / sectors_per_cluster;
 
-               cluster = ao_fat_cluster_seek(ao_file_dirent.cluster, cluster_offset);
+               cluster_distance = cluster_offset - ao_file_cluster_offset / bytes_per_cluster;
+
+               cluster = ao_fat_cluster_seek(ao_file_cluster, cluster_distance);
+
                if (!ao_fat_cluster_valid(cluster))
                        return 0xffffffff;
                ao_file_cluster = cluster;
@@ -392,16 +402,6 @@ ao_fat_current_sector(void)
 static void
 ao_fat_set_offset(uint32_t offset)
 {
-       
-       if (offset == 0) {
-               ao_file_cluster = ao_file_dirent.cluster;
-               ao_file_cluster_offset = 0;
-       }
-       else if (offset < ao_file_cluster_offset ||
-           ao_file_cluster_offset + bytes_per_cluster <= offset)
-       {
-               ao_file_cluster = 0;
-       }
        ao_file_offset = offset;
 }