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//! Iterative depth-first traversal of the object graph that assigns each
//! reachable node a DFS pre-order number and records its DFS-tree parent.
//! Feeds the SEMI-NCA dominator stage; the `vertex` permutation is rebuilt
//! lazily to keep it off the RSS peak.
/// Result of [`rpo_dfs`]: DFS pre-order numbering and tree structure for a
/// graph with a virtual root at index `n` whose children are `roots`; all
/// other edges come from the forward CSR (`fwd_off`, `fwd_tgt`).
///
/// No recursion is used — real heaps have millions of nodes.
///
/// # Indexing convention
/// Objects are 0..n-1; the virtual root is index `n`.
/// Mirrors the Java `RpoDfs` in hprof-analyzer with the root/object index
/// convention flipped.
pub struct RpoResult {
/// `parent_pre[i]` = pre-order number of the DFS-tree parent of the node
/// whose pre-order number is `i` (i.e. of `vertex[i]`).
/// `parent_pre[0]` = 0 (virtual root's parent is itself).
/// Length = number of reachable nodes + 1 (index 0 = virtual root),
/// lockstep with `vertex`.
pub parent_pre: Vec<u32>,
/// DFS pre-order number of each node. `u32::MAX` = unvisited.
/// Virtual root (index n) gets dfn 0; visited nodes get 1, 2, 3, ... in DFS pre-order.
/// Length = n + 1. Used by SEMI-NCA dominator.
pub dfn: Vec<u32>,
/// Inverse of `dfn`: `vertex[i]` = node whose pre-order number is `i`.
/// Length = number of reachable nodes + 1 (index 0 = virtual root).
///
/// NOT built during the DFS: at 514M nodes this 1.96GB array is idle
/// through the inbound 2b scan (the binding RSS peak — inb_flat + id_map +
/// dfn + parent_pre + in_cursors all resident). `rpo_dfs` returns this
/// EMPTY; the caller rebuilds it from `dfn` via [`rebuild_vertex`] AFTER
/// inbound.build, just before the dominator stage that actually reads it.
pub vertex: Vec<u32>,
}
/// Rebuild the `vertex` permutation (inverse of `dfn`) as a pure O(n) pass.
/// `count` = number of reachable nodes + 1 (== parent_pre.len()).
pub fn rebuild_vertex(dfn: &[u32], count: usize) -> Vec<u32> {
let mut vertex = vec![0u32; count];
for (node, &pre) in dfn.iter().enumerate() {
if pre != u32::MAX {
vertex[pre as usize] = node as u32;
}
}
vertex
}
/// Traverse the graph from the virtual root, assigning DFS pre-order numbers
/// (`dfn`) and DFS-tree parents (`parent_pre`). `vertex` is returned empty;
/// see [`RpoResult::vertex`] and [`rebuild_vertex`].
pub fn rpo_dfs(
n: usize,
roots: &[u32],
fwd_off: &[u32],
fwd_tgt: &crate::chunkvec::ChunkU32,
) -> RpoResult {
let vroot = n as u32;
let mut parent_pre: Vec<u32> = Vec::with_capacity(n + 1);
let mut dfn = vec![u32::MAX; n + 1];
let mut dfs_count: u32 = 0;
crate::trace::probe("rpo_dfs: after dfn+parent_pre alloc (before DFS loop)");
// Explicit stacks: parallel arrays (node, child_cursor)
let mut node_stack: Vec<u32> = Vec::with_capacity(1024);
let mut cursor_stack: Vec<usize> = Vec::with_capacity(1024);
// Push virtual root (pre-order number 0)
dfn[n] = dfs_count;
parent_pre.push(0); // virtual root's parent is itself (pre-order 0)
dfs_count += 1;
node_stack.push(vroot);
cursor_stack.push(0);
while !node_stack.is_empty() {
let top = *node_stack.last().unwrap();
let cursor = cursor_stack.last_mut().unwrap();
// Number of children for `top`
let child_count: usize = if top == vroot {
roots.len()
} else {
let v = top as usize;
(fwd_off[v + 1] - fwd_off[v]) as usize
};
let mut pushed = false;
if top == vroot {
// Virtual root's children come from the `roots` slice directly.
while *cursor < child_count {
let child = roots[*cursor];
*cursor += 1;
if child as usize > n {
continue;
}
if dfn[child as usize] == u32::MAX {
dfn[child as usize] = dfs_count;
parent_pre.push(dfn[top as usize]);
dfs_count += 1;
node_stack.push(child);
cursor_stack.push(0);
pushed = true;
break;
}
}
} else {
let v = top as usize;
let lo = fwd_off[v] as usize;
let hi = fwd_off[v + 1] as usize;
let parent_dfn = dfn[top as usize];
// Fast path: adjacency list fits in one chunk — iterate the slice
// directly without per-element shift/mask overhead.
if let Some(adj) = fwd_tgt.range_slice(lo + *cursor, hi) {
for &child in adj {
*cursor += 1;
if child as usize > n {
continue;
}
if dfn[child as usize] == u32::MAX {
dfn[child as usize] = dfs_count;
parent_pre.push(parent_dfn);
dfs_count += 1;
node_stack.push(child);
cursor_stack.push(0);
pushed = true;
break;
}
}
} else {
// Cross-chunk fallback: use individual get() calls.
while *cursor < child_count {
let child = fwd_tgt.get(lo + *cursor);
*cursor += 1;
if child as usize > n {
continue;
}
if dfn[child as usize] == u32::MAX {
dfn[child as usize] = dfs_count;
parent_pre.push(parent_dfn);
dfs_count += 1;
node_stack.push(child);
cursor_stack.push(0);
pushed = true;
break;
}
}
}
}
if !pushed {
// All children processed → node finishes, pop it.
node_stack.pop();
cursor_stack.pop();
}
}
RpoResult {
parent_pre,
dfn,
// Rebuilt by the caller via rebuild_vertex() after inbound.build.
vertex: Vec::new(),
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::chunkvec::ChunkU32;
fn make_fwd_tgt(v: Vec<u32>) -> ChunkU32 {
let mut c = ChunkU32::zeroed(v.len());
for (i, &x) in v.iter().enumerate() {
c.set(i, x);
}
c
}
#[test]
fn rpo_diamond() {
// 0→{1,2}, 1→3, 2→3; roots=[0]
let fwd_off = vec![0u32, 2, 3, 4, 4];
let fwd_tgt = make_fwd_tgt(vec![1u32, 2, 3, 3]);
let r = rpo_dfs(4, &[0u32], &fwd_off, &fwd_tgt);
// All 4 nodes reachable → each has a real pre-order number.
for v in 0..4usize {
assert_ne!(r.dfn[v], u32::MAX, "node {v} must be reachable");
}
// Pre-order from root 0: 0 first, then its subtree. Node 3 is reached
// via node 1 (0's first child) before node 2 is opened, so
// dfn[0] < dfn[1] < dfn[3] < dfn[2].
assert!(r.dfn[0] < r.dfn[1], "root 0 visited before node 1");
assert!(r.dfn[1] < r.dfn[3], "node 3 discovered via node 1");
assert!(r.dfn[3] < r.dfn[2], "node 3 visited before node 2 (DFS)");
}
#[test]
fn rpo_unreachable() {
// 0→1; node 2 unreachable; roots=[0]
let fwd_off = vec![0u32, 1, 1, 1];
let fwd_tgt = make_fwd_tgt(vec![1u32]);
let r = rpo_dfs(3, &[0u32], &fwd_off, &fwd_tgt);
// nodes 0,1 reachable; node 2 unreachable (no DFS number).
assert_ne!(r.dfn[0], u32::MAX);
assert_ne!(r.dfn[1], u32::MAX);
assert_eq!(r.dfn[2], u32::MAX);
}
}