Struct NavExtractor 

struct NavExtractor<'a> {
    input_fields: &'a [Field],
    resolver: &'a VarResolver<'a>,
    symbol: &'a str,
    prefix: &'a str,
    nav_slots: Vec<DefineSlot>,
    nav_fields: Vec<Field>,
}

Extracts row-pattern navigation functions (PREV/NEXT/FIRST/LAST) from a DEFINE predicate AST, replacing each with a synthetic placeholder column and recording the corresponding DefineSlot. Functions are handled because they do not bind as ordinary scalar functions; plain variable-qualified columns are left to bind normally and are mapped later.

Fields

input_fields: &'a [Field]

resolver: &'a VarResolver<'a>

symbol: &'a str

The symbol whose DEFINE predicate is being lowered — the only variable qualifier a physical PREV may carry (see NavExtractor::physical_col).

prefix: &'a str

Per-DEFINE prefix for the synthetic placeholder column names (kept unique across DEFINE items).

nav_slots: Vec<DefineSlot>

nav_fields: Vec<Field>

Implementations

impl NavExtractor<'_>

fn rewrite(&mut self, node: &mut AstExpr) -> RwResult<()>

fn nav_slot(&self, func: &Function) -> RwResult<DefineSlot>

Builds the DefineSlot for a navigation function call.

fn physical_col(&self, expr: &AstExpr) -> RwResult<usize>

Resolves a physical-navigation argument (col or var.col) to its input column index.

A variable qualifier is accepted only when it is the symbol being defined: under the standard’s running semantics PREV(B.col) inside DEFINE B reads from the row before the current candidate — exactly the physical previous row this engine navigates to. A qualifier naming ANOTHER variable anchors the read to that variable’s last mapped row instead (PREV(A.col) ≡ PREV(LAST(A.col, 0), 1)), which is different semantics this engine does not implement — silently treating it as the physical previous row would answer a question the author did not ask. An undeclared qualifier is plain wrong SQL.

fn var_col(&self, expr: &AstExpr) -> RwResult<(Vec<String>, usize)>

Resolves a logical-navigation argument (var.col) to its variable(s) and input column index.

fn col_idx(&self, name: &str) -> RwResult<usize>

fn parse_offset(&self, arg: &FunctionArg, name: &str) -> RwResult<usize>

fn offset_above_cap(name: &str, literal: &str) -> RwError

fn offset_not_a_positive_literal(name: &str) -> RwError