DOC: add a diagram to explain how circular buffers work
Also add some thoughts about the existing and new design.
Note: an earlier design used the names "head" and "tail" for both sides
of the buffer, but it appears awkward as these words may be understood
in two forms (feed by head, output by tail, or make the newcomers wait
at the tail of the queue). Also there were already a few functions in the
code making use of either terminology. So better avoid this terminology
and use "input" and "output" instead.
diff --git a/doc/internals/buffer-ops.fig b/doc/internals/buffer-ops.fig
new file mode 100644
index 0000000..5cca1b6
--- /dev/null
+++ b/doc/internals/buffer-ops.fig
@@ -0,0 +1,152 @@
+#FIG 3.2
+Portrait
+Center
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+4 1 0 50 -1 16 12 0.0000 4 165 2370 3150 855 Principle of the circular buffer\001
+4 1 0 50 -1 12 8 0.0000 4 90 600 1260 3960 buffer_*\001
+4 0 0 50 -1 12 8 0.0000 4 90 600 4950 3420 buffer_*\001
+4 1 0 50 -1 16 12 0.0000 4 165 1605 3150 1125 Current (since v1.3)\001
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+4 0 0 50 -1 0 10 0.0000 4 135 1860 6075 1755 Some http_msg fields point to\001
+4 0 0 50 -1 0 10 0.0000 4 120 2175 6075 1950 absolute locations within the buffer,\001
+4 0 0 50 -1 0 10 0.0000 4 135 2040 6075 2145 making realignments quite tricky.\001
+4 0 0 50 -1 0 10 0.0000 4 135 1890 6075 5400 http_msg owns a pointer to the\001
+4 0 0 50 -1 0 10 0.0000 4 135 2055 6075 5595 struct_buffer and only uses offsets\001
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+4 1 0 50 -1 12 8 0.5236 4 75 75 3870 7740 i\001
+4 1 0 50 -1 16 12 0.0000 4 180 1965 3150 4905 New design (1.5-dev9+)\001
+4 1 0 50 -1 12 8 0.0000 4 60 75 3150 7920 p\001