From 12e37d4335d611db0d27bfb999dd9b7a5b6f161e Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Wed, 21 Oct 2020 19:57:14 +0100
Subject: [PATCH] Introduce the 'true' and 'false' conditions

---
 README.md                        |  3 +++
 crsn/src/asm/instr/cond.rs       | 11 +++++++++++
 crsn/src/runtime/frame/status.rs |  2 ++
 examples/test_cond.csn           | 24 ++++++++++++++++++++++++
 4 files changed, 40 insertions(+)
 create mode 100644 examples/test_cond.csn

diff --git a/README.md b/README.md
index fbc4ec9..42604ae 100644
--- a/README.md
+++ b/README.md
@@ -98,6 +98,8 @@ These keywords (among others) are used in conditional branches to specify flag t
 - `nem`, `nempty` … Not empty
 - `eof` … EOF
 - `neof` … Not EOF
+- `true`, `always`, `else` … Always true
+- `false`, `never` … Always false
 
 # Syntax
 
@@ -207,6 +209,7 @@ Conditonal branches are written like this:
   then only one branch is taken - there is no fall-through.
 - The definition order is preserved, i.e. if the `inval` flag is to be checked, it should be done 
   before checking e.g. `nz`, which is, incidentally, true by default, because most flags are cleared by instructions that affects flags.
+- `else` can be used as a final choice of branch that will always be taken.
 
 ## Routines
 
diff --git a/crsn/src/asm/instr/cond.rs b/crsn/src/asm/instr/cond.rs
index c3b86d3..13ff6c9 100644
--- a/crsn/src/asm/instr/cond.rs
+++ b/crsn/src/asm/instr/cond.rs
@@ -106,6 +106,10 @@ pub enum Cond {
     Eof,
     /// Not empty
     NotEof,
+    // Always true, for eg. (else? ...)
+    True,
+    /// Always false
+    False,
 }
 
 pub fn parse_cond(text: &str, pos: &SourcePosition) -> Result<Cond, CrsnError> {
@@ -134,6 +138,8 @@ pub fn parse_cond(text: &str, pos: &SourcePosition) -> Result<Cond, CrsnError> {
         "neof" => Cond::NotEof,
         "ov" => Cond::Overflow,
         "nov" => Cond::NotOverflow,
+        "true" | "always" | "else" => Cond::True,
+        "false" | "never" => Cond::False,
         _ => {
             return Err(CrsnError::Parse(format!("Unknown cond: {}", text).into(), pos.clone()));
         }
@@ -174,6 +180,8 @@ impl Display for Cond {
             Cond::Valid => "valid",
             Cond::Eof => "eof",
             Cond::NotEof => "neof",
+            Cond::True => "true",
+            Cond::False => "false",
         })
     }
 }
@@ -214,6 +222,9 @@ impl Not for Cond {
 
             Cond::NotEof => Cond::Eof,
             Cond::Eof => Cond::NotEof,
+
+            Cond::True => Cond::False,
+            Cond::False => Cond::True,
         }
     }
 }
diff --git a/crsn/src/runtime/frame/status.rs b/crsn/src/runtime/frame/status.rs
index dec3d5a..286c897 100644
--- a/crsn/src/runtime/frame/status.rs
+++ b/crsn/src/runtime/frame/status.rs
@@ -115,6 +115,8 @@ impl StatusFlags {
             Cond::NotEmpty => !self.empty,
             Cond::Eof => self.eof,
             Cond::NotEof => !self.eof,
+            Cond::True => true,
+            Cond::False => false,
         }
     }
 
diff --git a/examples/test_cond.csn b/examples/test_cond.csn
new file mode 100644
index 0000000..b4a5208
--- /dev/null
+++ b/examples/test_cond.csn
@@ -0,0 +1,24 @@
+(
+  (ld r8 0)
+
+  (ld r0 1
+    (true? (add r8 1))
+  )
+
+  (sub r0 1
+    (pos? (fault))
+    (else? (add r8 1))
+  )
+
+  (nop
+    (always? (add r8 1))
+  )
+
+  (nop
+    (never? (fault))
+  )
+
+  (cmp r8 3
+    (ne? (fault))
+  )
+)