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--- language:langs:chp [2023/04/07 12:13] – [Channels in expressions] rajit
+++ language:langs:chp [2024/07/20 16:36] (current) – [Advanced expression syntax] rajit
@@ Line 86: / Line 86: @@
 In the published literature, non-deterministic selections are usually written using a thin bar ''|'' rather than the thick bar ''[]''. However, this can create some ambiguities in parsing. For example, consider the statement ''[ a -> y:=b | c | d -> skip ]''. This can be parsed in a number of ways, and it is not clear which one was the intended option.
+===== Arrays: dynamic v/s non-dynamic indices =====
+Suppose an array ''x'' has been declared as:
+<code act>
+int x[10];
+</code>
+Now when ''x'' is accessed in CHP, it could be accessed with an array index that is a run-time constant, or an index that is computed at run-time. For example, the CHP program
+<code act>
+chp {
+   ...
+   x[0] := x[0] + 1;
+   ...
+}
+</code>
+uses ''x'' with a constant index. As opposed to this, the program
+<code act>
+chp {
+   ...
+   x[i] := x[i] + 1;
+   ...
+}
+</code>
+uses ''x'' with an index that is computed using the run-time value of ''i''. This second category of arrays are referred to as //dynamic arrays//---not because the array size is dynamic, but because the element of the array accessed depends on a value that is computed by the circuit.  Such dynamic arrays have to be translated into memory structures, or other circuits where the element being accessed has to be specified at run-time. Arrays with constant references can be directly mapped to circuit implementations of asynchronous registers, since the element to be accessed can be determined statically.
 ===== Loops =====
@@ Line 130: / Line 156: @@
 ===== Advanced expression syntax =====
+==== Bitslice assignment ====
+For integer-valued variables, one can assign to some of the bits. The syntax for this is:
+<code act>
+ int<8> x;
+ ...
+ chp {
+   ...
+    x{4..2} := 4;
+   ...
+ }
+</code>
+This will update three bits of ''x'' based on the bit-pattern of the right hand side (in this case ''100'').
 ==== Channels in expressions ====
@@ Line 223: / Line 266: @@
 Essentially every channel variable ''V'' can be translated into ''(#V ? V : ERR)''. Note that this is a different translation compared to guards. Note that since probes are only allowed in guards of selection statements, negated probes are not possible in other contexts.
+===== Advanced channels =====
 ==== Exchange channels ====
@@ Line 235: / Line 280: @@
 Four-phase handshake channels involve two synchronizations. If you need to make this explicit in the CHP, the ''!'' (send) and ''?'' (receive) operators can be replaced with ''!+'' (first half of send), ''!-'' (second half of send), or ''?+'' (first half of receive), ''?-'' (second half of receive). The second half is purely synchronization and should not have any data.
-==== Syntactic replication ====
+===== Syntactic replication =====
 There are three common uses of syntactic replication in CHP, and the language has syntactic support for them. The first two correspond to scenarios where you have a number of items separated by semicolon or comma. The syntax for this follows the generic template for [[language:controlflow|syntactic replication]]:
@@ Line 258: / Line 303: @@
 Similarly, the semicolon can be replaced with a comma to specify parallel execution.
+Syntactic replication is also supported for guarded commands.  The statement
+<code act>
+[ v = 0 -> x := x  - v
+[] ( [] i : 3 : v = i+2 -> x := x + i
+]
+</code>
+is the same as
+<code act>
+[ v = 0 -> x := x - v
+[] v = 0+2 -> x := x + 0
+[] v = 1+2 -> x := x + 1
+[] v = 2+2 -> x := x  + 2
+]
+</code>
+This construct can be used to work around one of the current restrictions of CHP. A natural way to write a merge process in ACT is:
+<code act>
+*[ C?c; I[c]?d; O!d ]
+</code>
+This will error out because the CHP has a //dynamic// channel access---i.e. the channel accessed is computed at run-time. While this is supported for int arrays, for example, that is because there is an efficient compilation mechanism for large int arrays (via memory macros). To work around this restriction, one can use:
+<code act>
+*[ C?c; [ ([] i : N : c = i -> I[i]?d) ]; O!d ]
+</code>
+where ''N'' is the number of channels in the ''I'' array. This approach ensures that index for the array ''I[]'' is always a constant.
 ====== The chp-txt sublanguage ======