diff -r a60d2f12ef93 -r 22a64e826ee7 src/Logger.jl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Logger.jl	Tue Dec 07 11:41:07 2021 +0200
@@ -0,0 +1,96 @@
+#
+# This module implements logging of intermediate computational results for
+# generating convergence graphs, etc.
+#
+
+"""
+Logging routines for intermediate computational results.
+Includes `Log`, `log!`, `if_log!`, and `write_log`.
+"""
+module Logger
+
+import DelimitedFiles: writedlm
+
+##############
+# Our exports
+##############
+
+export Log,
+       log!,
+       if_log!,
+       write_log
+
+##########
+# Logging
+##########
+
+"""
+`struct Log{T}`
+
+A log of items of type `T` along with log configuration.
+The constructor takes no arguments;  create the log with `Log{T}()` for `T` your
+data type, e.g., `Float64`.
+"""
+struct Log{T}
+    log :: Dict{Int, T}
+
+    Log{T}() where T = new{T}(Dict{Int, T}())
+end
+
+"""
+`if_log!(f :: Functdion, log :: Log{T}, i :: Int)`
+
+If based on the log settings, `i` is a verbose iteration, store the value of `f()`
+in the log at index `i`. Typically to be used with `do`-notation:
+
+```julia
+if_log!(log, iteration) do
+    # calculate value to be logged
+end
+```
+"""
+function if_log!(f :: Function, log :: Log{T}, i :: Int) where T
+    if mod(i, log.verbose_iterations)==0
+        log!(f, log, i)
+        #printstyled("$(i): $(val)\n", color=:light_black)
+    end
+end
+
+"""
+`log!(f :: Function, log :: Log{T}, i :: Int) `
+
+Store the value `f()` in the log at index `i`.
+"""
+function log!(f :: Function, log :: Log{T}, i :: Int) where T
+    val = f()
+    push!(log.log, i => val)
+end
+
+##############
+# Data export
+##############
+
+"""
+`write_log(filename :: AbstractString, log :: Log{T})`
+
+Write a `Log{T}` as a CSV file using `DelimitedFiles`.
+If T is a structural type, the field names are used as header fields.
+Otherwise a single `value` field is attempted to be written.
+"""
+function write_log(filename :: AbstractString, log :: Log{T}) where T
+    k = fieldnames(T)
+    @assert(:iter ∉ k)
+    
+    open(filename, "w") do io
+        # Write header
+        writedlm(io, isempty(k) ? [:iter :value] : ((:iter, k...),))
+        # Write values
+        iters = sort(collect(keys(log.log)))
+        for i ∈ iters
+            v = log.log[i]
+            writedlm(io, isempty(k) ? [i v] : ((i, (getfield(v, j) for j ∈ k)...),))
+        end
+    end
+end
+
+end # module