#!/usr/bin/env bash
# What each JDK decides by default, and what the flags do. Every line is the JVM's own answer
# (java -XX:+PrintFlagsFinal), not something inferred from documentation.
source "$(dirname "$0")/lib.sh"

show() {  # label jdk flags...     (set CG_MEM/CG_CPUS to run inside a container-style cgroup)
  local label="$1" jh="$2"; shift 2
  local out cg=""
  if [ -n "${CG_MEM:-}" ]; then
    cg="fl$$-$RANDOM"; cg_create "$cg" "$CG_MEM" "${CG_CPUS:-2}"
    out=$(bash -c "$CG_JOIN" _ "$cg" "$jh/bin/java" "$@" -XX:+PrintFlagsFinal -version 2>&1)
    cg_remove "$cg"
  else
    out=$("$jh/bin/java" "$@" -XX:+PrintFlagsFinal -version 2>&1)
  fi
  if ! echo "$out" | grep -q 'UseCompactObjectHeaders'; then
    printf '%-52s -> JVM refused to start:\n' "$label"; echo "$out" | head -3 | sed 's/^/      /'; return
  fi
  local ch gc heap
  ch=$(echo "$out" | awk '$2=="UseCompactObjectHeaders"{print $4}')
  gc=$(echo "$out" | awk '($2=="UseG1GC"||$2=="UseSerialGC"||$2=="UseParallelGC"||$2=="UseZGC") && $4=="true"{print $2}' | tr '\n' ' ')
  heap=$(echo "$out" | awk '$2=="MaxHeapSize"{printf "%d", $4/1048576}')
  printf '%-52s -> compact=%-5s collector=%-13s MaxHeapSize=%sMB\n' "$label" "$ch" "$gc" "$heap"
  echo "$out" | grep -E '^(OpenJDK 64-Bit Server VM warning|Error|Unrecognized)' | sed 's/^/      JVM says: /' | head -2
}

for v in 25 26 27; do
  jh_var="JDK$v"; jh="${!jh_var}"
  echo "== JDK $v: $("$jh/bin/java" -version 2>&1 | sed -n 2p)"
  show "default (this machine: 2 CPUs)"             "$jh"
  show "-XX:+UseCompactObjectHeaders"               "$jh" -XX:+UseCompactObjectHeaders
  show "-XX:-UseCompactObjectHeaders"               "$jh" -XX:-UseCompactObjectHeaders
  echo
done

echo "== The same JDKs in a container: 1 GB memory limit, 1 CPU (a cgroup, set the way Docker sets it)"
for v in 25 26 27; do
  jh_var="JDK$v"; jh="${!jh_var}"
  CG_MEM=$((1024*1024*1024)) CG_CPUS=1 show "JDK $v, 1 GB, 1 CPU"                    "$jh"
  CG_MEM=$((1024*1024*1024)) CG_CPUS=1 show "JDK $v, 1 GB, 1 CPU, MaxRAMPercentage=75" "$jh" -XX:MaxRAMPercentage=75
done
echo
echo "== 1 GB, 2 CPUs"
for v in 26 27; do
  jh_var="JDK$v"; jh="${!jh_var}"
  CG_MEM=$((1024*1024*1024)) CG_CPUS=2 show "JDK $v, 1 GB, 2 CPUs"                    "$jh"
done
echo

echo "== -XX:MaxRAM: the flag many people used to fake a container limit"
show "JDK 26 -XX:MaxRAM=1g"                         "$JDK26" -XX:MaxRAM=1g
show "JDK 27 -XX:MaxRAM=1g"                         "$JDK27" -XX:MaxRAM=1g
echo

echo "== Compact headers with each collector on JDK 27"
for gc in UseG1GC UseParallelGC UseSerialGC UseZGC; do
  show "JDK 27 -XX:+$gc" "$JDK27" -XX:+$gc
done
echo

echo "== Compressed pointers on JDK 27"
show "JDK 27 -XX:-UseCompressedClassPointers"       "$JDK27" -XX:-UseCompressedClassPointers
show "JDK 27 -XX:-UseCompressedOops"                "$JDK27" -XX:-UseCompressedOops
show "JDK 27 -Xmx32g (a large explicit heap)" "$JDK27" -Xmx32g