Allow changing decoration (#632)

* Allow changing decoration

* Add details in docstring

Author: OlivierHnt

src/intervals/construction.jl

@@ -236,7 +236,6 @@ end
 # note: `isless`, and hence `<`, `min` and `max`, are automatically defined
 
 function decoration(x::BareInterval)
-    isnai(x) && return ill
     isempty_interval(x) && return trv
     isunbounded(x) && return dac
     return com
@@ -302,6 +301,31 @@ end
 bareinterval(::Type{T}, x::Interval) where {T} = bareinterval(T, bareinterval(x))
 decoration(x::Interval) = x.decoration
 
+"""
+    setdecoration(x::Interval, d::Decoration)
+
+Return the interval `bareinterval(x)` with decoration `d`, with the following
+exceptions:
+- if `d == ill`, then the output is an NaI
+- if `isempty_interval(bareinterval(x))` and `d != ill`, then the output has
+decoration `trv`
+- if `isunbounded(bareinterval(x))` and `d == com`, then the output has
+decoration `dac`
+
+!!! danger
+    Since misuse of this function can deeply corrupt code, its usage is
+    **strongly discouraged**.
+
+Implement the `setDec` function of the IEEE Standard 1788-2015 (Section 11.5.2).
+"""
+function setdecoration(x::Interval{T}, d::Decoration) where {T<:NumTypes}
+    d == ill && return nai(T)
+    bx = bareinterval(x)
+    isempty_interval(bx) && return _unsafe_interval(bx, trv, isguaranteed(x))
+    (isunbounded(bx) & (d == com)) && return _unsafe_interval(bx, dac, isguaranteed(x))
+    return _unsafe_interval(bx, d, isguaranteed(x))
+end
+
 """
     isguaranteed(x::BareInterval)
     isguaranteed(x::Interval)

test/interval_tests/consistency.jl

@@ -34,6 +34,13 @@
         @test isequal_interval(interval(0, 1) - emptyinterval(a), emptyinterval(a))
         @test isequal_interval(interval(0, 1) * emptyinterval(a), emptyinterval(a))
         @test isequal_interval(a * interval(0), zero(a))
+
+        @test decoration(IntervalArithmetic.setdecoration(interval(1, 2), ill)) == ill
+        @test decoration(IntervalArithmetic.setdecoration(emptyinterval(), com)) == trv
+        @test decoration(IntervalArithmetic.setdecoration(interval(1, Inf), com)) == dac
+
+        @test !isnai(IntervalArithmetic.setdecoration(interval(NaN), com))
+        @test decoration(IntervalArithmetic.setdecoration(interval(NaN), com)) == trv
     end
 
     @testset "real interface" begin