Increase speed a thousand fold by caching

fixture/endless.sh

@@ -2,21 +2,47 @@
 
 # sixel endless mode
 # Should print an endless sine curve, abort with Ctrl-C.
+# A genuine VT340 terminal updates this continuously. 
+
+# Always send ST to end sixel mode when script exits
+ST=$'\e\\'
+trap 'echo -en "$ST"'  EXIT	
 
 period=200
 amplitude=50
 
-sixels=(@\$ A\$ C\$ G\$ O\$ _\$-)
-pi=$(echo "scale=10; 4*a(1)" | bc -l)
-run=true
-trap run=false INT
+sixels=(@$ A$ C$ G$ O$ _\$-)
+pi=3.141592653589793238
+
+# Number of samples (m*period) must be divisble by both $period and 6. 
+declare -i m=1
+if ! factor $period 2>&- | cut -d: -f2- | grep -q -w 2; then
+    m=m*2
+fi
+if ! factor $period 2>&- | cut -d: -f2- | grep -q -w 3; then
+    m=m*3
+fi
 
+# Start sixels graphics string
 echo -ne "\x1bP0;0;0q\"1;1#1;2;100;0;0#1"
-y=0
-while $run
+
+# Cache the calculations in an array for speed
+for (( y=0; y < m*period; y++ ))
+do
+  x=$(echo "x=s(2*${pi}*${y}/${period})*${amplitude}+2*${amplitude}+0.5; scale=0; x/1" | bc -l)
+  echo -ne "!${x}?${sixels[$((y%6))]}"		   	# 1.8 KBps (uncached)
+  f[y]=$(echo -ne "!${x}?${sixels[$((y%6))]}")		# 1.8 MBps (cached elts)
+  g+=$(echo -ne "!${x}?${sixels[$((y%6))]}")		# 88 MBps (string)
+done
+
+while :
 do
-  x=$(echo "s(2*${pi}*${y}/${period})*${amplitude}+2*${amplitude}+0.5" | bc -l)
-  echo -ne "!${x%%.*}?${sixels[$((y%6))]}"
-  (( y++ ))
+    for y in ${!f[@]}; do echo -n "${f[y]}"; done 	# 1.8 MBps (cached elt)
+#    echo -n "${f[*]}"					# 48 MBps (all elts)
+#    echo -n "$g"					# 88 MBps (string)
 done
-echo -e "\x1b\\"
+
+
+# NOTES
+#
+# Simple way to measure the speed is `./endless.sh | pv >/dev/null`