From 588127bf4105d8d4629304521abc3d992326a63a Mon Sep 17 00:00:00 2001
From: Nadrieril
Date: Thu, 18 Apr 2019 19:34:38 +0200
Subject: subst_shift in a single pass

---
 dhall/src/typecheck.rs    | 17 ++++++-------
 dhall_core/src/context.rs | 14 +++++++++++
 dhall_core/src/core.rs    | 62 +++++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 78 insertions(+), 15 deletions(-)

diff --git a/dhall/src/typecheck.rs b/dhall/src/typecheck.rs
index 5aaeb08..186384d 100644
--- a/dhall/src/typecheck.rs
+++ b/dhall/src/typecheck.rs
@@ -44,9 +44,9 @@ impl<'a> Normalized<'a> {
     fn unroll_ref(&self) -> &Expr<X, X> {
         self.as_expr().as_ref()
     }
-    fn shift(&self, delta: isize, var: &V<Label>) -> Self {
+    fn shift0(&self, delta: isize, label: &Label) -> Self {
         // shift the type too ?
-        Normalized(shift(delta, var, &self.0), self.1.clone(), self.2)
+        Normalized(shift0(delta, label, &self.0), self.1.clone(), self.2)
     }
 }
 impl Normalized<'static> {
@@ -86,10 +86,10 @@ impl<'a> Type<'a> {
             Cow::Owned(e) => Ok(Cow::Owned(e.into_expr().unroll())),
         }
     }
-    fn shift(&self, delta: isize, var: &V<Label>) -> Self {
+    fn shift0(&self, delta: isize, label: &Label) -> Self {
         use TypeInternal::*;
         Type(match &self.0 {
-            Expr(e) => Expr(Box::new(e.shift(delta, var))),
+            Expr(e) => Expr(Box::new(e.shift0(delta, label))),
             Const(c) => Const(*c),
             SuperType => SuperType,
         })
@@ -378,9 +378,7 @@ fn type_with(
     let ret = match e.as_ref() {
         Lam(x, t, b) => {
             let t = mktype(ctx, t.clone())?;
-            let ctx2 = ctx
-                .insert(x.clone(), t.clone())
-                .map(|e| e.shift(1, &V(x.clone(), 0)));
+            let ctx2 = ctx.insert(x.clone(), t.clone()).map(|e| e.shift0(1, x));
             let b = type_with(&ctx2, b.clone())?;
             Ok(RetExpr(Pi(
                 x.clone(),
@@ -395,9 +393,8 @@ fn type_with(
                 mkerr(InvalidInputType(ta.into_normalized()?)),
             );
 
-            let ctx2 = ctx
-                .insert(x.clone(), ta.clone())
-                .map(|e| e.shift(1, &V(x.clone(), 0)));
+            let ctx2 =
+                ctx.insert(x.clone(), ta.clone()).map(|e| e.shift0(1, x));
             let tb = type_with(&ctx2, tb.clone())?;
             let kB = ensure_is_const!(
                 &tb.get_type()?,
diff --git a/dhall_core/src/context.rs b/dhall_core/src/context.rs
index 877843d..b500226 100644
--- a/dhall_core/src/context.rs
+++ b/dhall_core/src/context.rs
@@ -43,6 +43,20 @@ impl<K: Hash + Eq + Clone, T> Context<K, T> {
                 .collect(),
         )
     }
+
+    pub fn lookup_all<'a>(&'a self, k: &K) -> impl Iterator<Item = &T> {
+        self.0.get(k).into_iter().flat_map(|v| v.iter())
+    }
+
+    pub fn iter<'a>(&'a self) -> impl Iterator<Item = (&K, &T)> {
+        self.0
+            .iter()
+            .flat_map(|(k, vs)| vs.iter().map(move |v| (k, v)))
+    }
+
+    pub fn iter_keys<'a>(&'a self) -> impl Iterator<Item = (&K, &Vec<T>)> {
+        self.0.iter()
+    }
 }
 
 impl<K: Hash + Eq + Clone, T: Clone> Context<K, T> {
diff --git a/dhall_core/src/core.rs b/dhall_core/src/core.rs
index aeb6f23..56ab66c 100644
--- a/dhall_core/src/core.rs
+++ b/dhall_core/src/core.rs
@@ -1,7 +1,9 @@
 #![allow(non_snake_case)]
 use std::collections::BTreeMap;
+use std::collections::HashMap;
 use std::rc::Rc;
 
+use crate::context::Context;
 use crate::visitor;
 use crate::*;
 
@@ -486,6 +488,7 @@ fn shift_var(delta: isize, var: &V<Label>, in_expr: &V<Label>) -> V<Label> {
 /// capture by shifting variable indices
 /// See https://github.com/dhall-lang/dhall-lang/blob/master/standard/semantics.md#shift
 /// for details
+///
 pub fn shift<N, E>(
     delta: isize,
     var: &V<Label>,
@@ -505,6 +508,41 @@ pub fn shift<N, E>(
     }
 }
 
+pub fn shift0<N, E>(
+    delta: isize,
+    label: &Label,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    shift(delta, &V(label.clone(), 0), in_expr)
+}
+
+fn shift0_multiple<N, E>(
+    deltas: &HashMap<Label, isize>,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    shift0_multiple_inner(&Context::new(), deltas, in_expr)
+}
+
+fn shift0_multiple_inner<N, E>(
+    ctx: &Context<Label, ()>,
+    deltas: &HashMap<Label, isize>,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    use crate::ExprF::*;
+    match in_expr.as_ref() {
+        Var(V(y, m)) if ctx.lookup(y, *m).is_none() => {
+            let delta = deltas.get(y).unwrap_or(&0);
+            rc(Var(V(y.clone(), add_ui(*m, *delta))))
+        }
+        _ => in_expr.map_subexprs_with_special_handling_of_binders(
+            |e| shift0_multiple_inner(ctx, deltas, e),
+            |l: &Label, e| {
+                shift0_multiple_inner(&ctx.insert(l.clone(), ()), deltas, e)
+            },
+        ),
+    }
+}
+
 /// Substitute all occurrences of a variable with an expression, and
 /// removes the variable from the environment by shifting the indices
 /// of other variables appropriately.
@@ -517,16 +555,30 @@ pub fn subst_shift<N, E>(
     var: &V<Label>,
     value: &SubExpr<N, E>,
     in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    subst_shift_inner(&HashMap::new(), var, value, in_expr)
+}
+
+fn subst_shift_inner<N, E>(
+    ctx: &HashMap<Label, isize>,
+    var: &V<Label>,
+    value: &SubExpr<N, E>,
+    in_expr: &SubExpr<N, E>,
 ) -> SubExpr<N, E> {
     use crate::ExprF::*;
+    let V(x, n) = var;
+    let dn = ctx.get(x).unwrap_or(&0);
+    let actual_var = V(x.clone(), add_ui(*n, *dn));
     match in_expr.as_ref() {
-        Var(v) if v == var => SubExpr::clone(value),
-        Var(v) => rc(Var(shift_var(-1, var, v))),
+        Var(v) if v == &actual_var => shift0_multiple(ctx, value),
+        Var(v) => rc(Var(shift_var(-1, &actual_var, v))),
         _ => in_expr.map_subexprs_with_special_handling_of_binders(
-            |e| subst_shift(var, &value, e),
+            |e| subst_shift_inner(ctx, var, value, e),
             |l: &Label, e| {
-                let vl = V(l.clone(), 0);
-                subst_shift(&shift_var(1, &vl, var), &shift(1, &vl, value), e)
+                let mut ctx = ctx.clone();
+                let count = ctx.entry(l.clone()).or_insert(0);
+                *count += 1;
+                subst_shift_inner(&ctx, var, value, e)
             },
         ),
     }
-- 
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